[Proposal] Database Migrations;

Summary: The database migrations now can be created using the methods on
the migration struct. it does not break any existing migration features.
it upgrades the migration struct and adds few more struct types so that
the migrations can be efficiently generated for create, alter, reverse,
drop.

Current Features:
* Supports creation of columns
   * `m.NewCol("name").SetDataType("VARCHAR(10)").SetNullable("true")`
   * **NOTE** `SetNullable` & `SetDefault` methods should not be called on
   same column for consistency
* Supports addition of primary keys
   * `m.PriCol("id").SetDataType("INT(10)").SetNullable("true")`
   * **NOTE** `setAuto(true)` can be only called on Primary keys
* Supports addition of unique keys
   * `m.UniCol("unique_index","column_name").SetDataType("VARCHAR(23)").SetNullable("true")`
   * **NOTE** `UniCol` can be called again with the same index name to
   add column to the index
* Supports rename of columns
   * `m.RenameColumn("from_name","to_name")`
   * Allows standard column methods and methods such that, `SetOldDefault` allows
   reversibility of renames

* TODO:
   * ForeignKey

Signed-off-by: Gnanakeethan Balasubramaniam <gnanakeethan@gmail.com>

migration/migration.go

@@ -52,6 +52,25 @@ type Migrationer interface {
 	GetCreated() int64
 }
 
+type Migration struct {
+	sqls           []string
+	Created        string
+	TableName      string
+	Engine         string
+	Charset        string
+	ModifyType     string
+	Columns        []*Column
+	Indexes        []*Index
+	Primary        []*Column
+	Uniques        []*Unique
+	Foreigns       []*Foreign
+	Renames        []*RenameColumn
+	RemoveColumns  []*Column
+	RemoveIndexes  []*Index
+	RemoveUniques  []*Unique
+	RemoveForeigns []*Foreign
+}
+
 var (
 	migrationMap map[string]Migrationer
 )
@@ -60,12 +79,6 @@ func init() {
 	migrationMap = make(map[string]Migrationer)
 }
 
-// Migration the basic type which will implement the basic type
-type Migration struct {
-	sqls    []string
-	Created string
-}
-
 // Up implement in the Inheritance struct for upgrade
 func (m *Migration) Up() {
 
@@ -76,6 +89,11 @@ func (m *Migration) Down() {
 
 }
 
+func (m *Migration) Migrate(migrationType string) {
+	m.ModifyType = migrationType
+	m.sqls = append(m.sqls, m.GetSQL())
+}
+
 // SQL add sql want to execute
 func (m *Migration) SQL(sql string) {
 	m.sqls = append(m.sqls, sql)