Add Travis-CI, and fix issues with transactions

Hi,

I've encountered several problems when running the unit tests: some `Set` tests would run indefinitely because they would never go out of a `WATCH` / `MULTI` / `EXEC` failed transaction loops. After investigation, I found out that some operations were incorrectly realised outside of the expected transaction because of misleading truth value testing on `redis.Pipeline` objects.

I hope I didn't miss any, and added a `.travis.yml` file in the proces (which should close #16 as soon as you add your project to Travis-CI).

Thank you for your work,
Arnaud