Add support for migrations using alembic

[ankitchiplunkar]

The database has already been created and i want to add support for database migrations using alembic.
This is a problem because:

1. Alembic or any other sqlalchemy migrations library supports database migrations from the first day.
2. Even we recreate my database what will happen to people who have already started syncing their database. We need to come up of someway that people can still migrate and preserve their synced data.

Supporting links: https://groups.google.com/forum/#!topic/sqlalchemy-alembic/2HJ9J6PiQsk

[ankitchiplunkar]

Completed this issue using pull request #13