Add support for 'deploy_via=copy'; revamp tasks with class-style

Hi Diego, fabistrano is the software I was looking for, thank you for it.

But I need more features, e.g. the "deploy_via=copy" I just added, which can checkout project from remote repo to local, then copy to servers.

Besides, I rewrite all fab tasks with classic-style (http://docs.fabfile.org/en/latest/usage/tasks.html#task-subclasses). Because, from my viewpoint, classes are more flexible and extensible than functions, which makes future development easier. (Base on this change, I change "with_defaults" decorator to "set_defaults" function)

I made a lot changes to fabistrano, and going to make probably more (e.g. multi-stage support). Would you like to merge my changes, or I'd better go with my folk?