Add cequel:shell script (similar to rails dbconsole)

**Background**
Rails provides a very useful rake task called ```rails dbconsole``` that launches mysql shell to the mysql server specified in your database.yml.
I found this very useful when connecting to remote mysql servers (much easier than ad-hocing the shell command myself). 
I think it would be cool if a similar task existed for Cassandra. This PR creates such task in the ```cequel``` namespace. 
**NOTE:** I've discovered that cequel-migrations-rails ignores the ```hosts``` property in cequel.yml. This appears to be a bug which I can fix for you guys.

**Regression**
Couldn't find any specs for the existing rake tasks so didn't create new specs for this one. If would like to see specs just say the word and I'll add.

Thanks!