Code refactoring: extract common parts from backends

[canasdiaz]

@sduenas suggested to improve the way backends are written. The idea of this enhancement is to study what the common parts are and move them to the shared code/objects.

[brainwane]

I've started working on this. It'll be in a branch in my fork. I aim to have something worth reviewing this week, even if it's just a small advance on the current situation.

[brainwane]

Sorry, this turned out to be more difficult than I thought, so I think I will aim on just fixing functionality bugs for now.

[canasdiaz]

Don't worry @brainwane ;)

[brainwane]

For future reference: right now, the Bugzilla, GitHub, Jira, and Launchpad backends use the Backend class (e.g., class LPBackend(Backend)); Allura, Gerrit, Google Code, Redmine, and Sourceforge backends don't (e.g., class Allura()).