Added schema autoupdate feature

When you add new columns or entities, you need to support your
OpenHelper with migration code (or use DevOpenHelper that just drops all
user's data).
This feature allows to avoid coding such migration by using "since" on
Entity or Entity Fields
1. Added annotation @Since for fields that allows to autogenerate
migration code for columns
2. Added "since" for @Entity annotation
3. Small refactoring for compatibility with old-style projects - gradle
is supported, but not required anymore
Small adjustments to the pull-request 1.