db migrations and sample migration - issue #62

Added alembic, when testing or upgrading make sure you pip install alembic. 

There is a sample migration owner_id on bundles. To run migrations after installing alembic just run :
 
    alembic upgrade head


I have also added this and more info to the readme. 

More info on alembic  https://pypi.python.org/pypi/alembic/0.6.3

It is written by the same guy who wrote sqlalchemy core. Please see the bottom about SQLite problems with ALTER and shortcomings. 

>Don't break our necks over SQLite's inability to ALTER things. SQLite has almost no support for table or column alteration, and this is likely intentional. Alembic's design is kept simple by not contorting its core API around these limitations, understanding that SQLite is simply not intended to support schema changes. While Alembic's architecture can support SQLite's workarounds, and we will support these features provided someone takes the initiative to implement and test, until the SQLite developers decide to provide a fully working version of ALTER, it's still vastly preferable to use Alembic, or any migrations tool, with databases that are designed to work under the assumption of in-place schema migrations taking place.

fixes issue #62 
change path to codalab_env/bin/alembic (also below)