Add support for installing plugins via VCS

``` python
use_plugin("vcs:git+git://github.com/...", plugin_name="plugin_name")
use_plugin("vcs:git+ssh://github.com/...", plugin_name="plugin_name")
```

This translates to `pip install --upgrade git+git://github.com/...`

@mriehl review please. I'm going to implement it as stated but if you find a problem I'll adjust later on.
