Update source-paths, dependencies, and repos automatically

Hi Toby,

Thank you for the great plugin.  I had a couple issues with `lein-git-deps`, though and tried to fix them in this PR.

Here is what I changed:

  - It is no longer required to manually add :source-paths.  A project middleware takes care of this now
  - It is no longer required to call `lein git-deps`, the repos and its dependencies will automatically be downloaded (also through the middleware)
  - The hook on `lein-deps` was not working in Leinigen 2.x, I fixed that as well
  - Previously Leinigen did not take over the dependency's dependencies and you had to add them manually to the parent project. This is now also fixed.

Please let me know if I should change anything in the code. I am happy to fix it if it could be more idiomatic, does not follow the proper coding style, etc.


Thanks,
Torsten