Add paginate methods for query.

API same as [Kaminari](https://github.com/kaminari/kaminari)

before:

```go
page, _ := strconv.Atoi(ctx.Query("page"))
if page <= 0 {
        page = 1
}
perPage, _ := strconv.Atoi(ctx.Query("size"))
if perPage <= 0 {
        perPage = 20
}
offset := (page - 1) * perPage
scope := DB.Where("name = ?", "jinzhu").Offset(offset).Limit(perPage)
```

after:

```go
scope := DB.Where("name = ?", "jinzhu").MaxPerPage(50).
        Page(ctx.Query("page")).Per(ctx.Query("size"))
```

search.go

@@ -2,6 +2,11 @@ package gorm
 
 import (
 	"fmt"
+	"strconv"
+)
+
+const (
+	defaultPerPage = 20
 )
 
 type search struct {
@@ -19,6 +24,9 @@ type search struct {
 	preload          []searchPreload
 	offset           interface{}
 	limit            interface{}
+	page             interface{}
+	perPage          interface{}
+	maxPerPage       int
 	group            string
 	tableName        string
 	raw              bool
@@ -92,6 +100,52 @@ func (s *search) Offset(offset interface{}) *search {
 	return s
 }
 
+func (s *search) Per(perPage interface{}) *search {
+	s.perPage = perPage
+	s.pageToOffsetLimit()
+	return s
+}
+
+func (s *search) Page(page interface{}) *search {
+	s.page = page
+	s.pageToOffsetLimit()
+	return s
+}
+
+func (s *search) MaxPerPage(maxPerPage int) *search {
+	s.maxPerPage = maxPerPage
+	s.pageToOffsetLimit()
+	return s
+}
+
+func (s *search) pageToOffsetLimit() *search {
+	page, err := strconv.Atoi(fmt.Sprintf("%v", s.page))
+	if err != nil || page <= 0 {
+		page = 1
+	}
+
+	perPage, err := strconv.Atoi(fmt.Sprintf("%v", s.perPage))
+	if err != nil || perPage <= 0 {
+		perPage = defaultPerPage
+	}
+
+	if perPage > s.maxPerPage && s.maxPerPage > 0 {
+		perPage = s.maxPerPage
+	}
+
+	offset := (page - 1) * perPage
+	if offset < 0 {
+		offset = 0
+	}
+
+	s.page = page
+	s.perPage = perPage
+	s.offset = offset
+	s.limit = perPage
+
+	return s
+}
+
 func (s *search) Group(query string) *search {
 	s.group = s.getInterfaceAsSQL(query)
 	return s

search_test.go

@@ -28,3 +28,41 @@ func TestCloneSearch(t *testing.T) {
 		t.Errorf("selectStr should be copied")
 	}
 }
+
+func TestPage(t *testing.T) {
+	s := new(search)
+	s.Where("name = ?", "jinzhu").Order("id").Page(1).Per(17)
+	if s.page != 1 {
+		t.Error("page should be 1")
+	}
+	if s.perPage != 17 {
+		t.Error("perPage should be 17")
+	}
+	if s.limit != 17 {
+		t.Error("limit should be 17")
+	}
+	if s.offset != 0 {
+		t.Error("offset should be 0")
+	}
+
+	s = new(search)
+	s.Where("name = ?", "jinzhu").Order("id").Per(17).Page(2)
+	if s.offset != 17 {
+		t.Error("offset should be 17")
+	}
+
+	s = new(search).Page(0).Per(0)
+	if s.page != 1 || s.perPage != defaultPerPage {
+		t.Error("page should be 1, perPage should be default per page")
+	}
+
+	s = new(search).Page("-3").Per("0")
+	if s.page != 1 || s.perPage != defaultPerPage || s.offset != 0 || s.limit != defaultPerPage {
+		t.Error("page should be 1, offset should be 0, perPage, limit should be default per page")
+	}
+
+	s = new(search).MaxPerPage(50).Page(2).Per("60")
+	if s.page != 2 || s.perPage != 50 || s.offset != 50 || s.limit != 50 {
+		t.Error("page should be 1, offset, limit, perPage should be 50")
+	}
+}