Enhanced, cleaner driver gateway

New Error type, and better error management.
Cleaner interface, moved to utils (pave the way for hipache-core / hipache-utils)
Enhanced readme
Better factory tests
Be consistent with `error` vs. `IDriver.ERROR`
Fix redis tests
Use hipache script instead of invoking node

... hence honor gc settings
typo fix
Merge pull request #119 from dotcloud/drivers-enhance

Drivers enhance