Improved PropEr testing and Rebar-ed the Erlang driver

This pull request introduces two major changes:

1. The Erlang driver for CakeDB now use rebar as build tool

2. All PropEr property-based testing is moved from the CakeDB app to the Erlang driver app. The resulting testing is more efficient and the code base is cleaner.

The README file in cakedb/drivers/cakedb_erlang_driver/ shows how to use rebar for the driver as well as how to run the PropEr tests.

I did not add cakedb_erlang_driver as a dependency to CakeDB because I could think of scenarios where a user would want to have one but not the other. If I'm not correct we can easily add cakedb_erlang_driver as a dependency in cakedb/rebar.config.

All comments welcome! :)