Rudimentary support for the git protocol in the downloader
Use same syntax as PIP to install from VCS URLs

The user has to specify a prefix that contains the name of the VCS
command and a the plus (`+`) sign. Just like this:

  $ curd install git+git@github.com:clarete/forbiddenfruit.git

Everything after the plus sign will be forwarded directly to the `git`
executable.
Improve the error message for unknown protocols installing vcs links
Support for most common VCS links. Closes #9