Adds VCS urls support.

Now, requirements, containing VCS urls could be compiled. During compilation, pip-compile will checkout sources, get all dependencies and pin spec to exact repository revision.

Pay attention, that some cram tests were broken even before I implemented this feature. I tried to fix one of them, but it seems too complex and I leaved it as with one minor fix — a function, removing `.pip-tools` before each call to `pip-compile`.