Index information in TableSchema

See https://github.com/yiisoft/yii2/issues/797#issuecomment-31102969

This is a draft for the definitive implementation.
Only for Mysql.
If discussion is positive, I could work for other implementations too (pgsql and sqlite, maybe).

**Some notes:**
* In **MySql** the way to search for indexes and unique keys could use the same `regexp`, so the same method could be used, but ketp them separated for readability.
* In **MySql** indexes are called keys, but left them as indexes because is more universal