Add support for Git-based NetworkSources

[flexi-framework]

Since Git is omnipresent in most development cycles and Julia itself is based all around Git, I was rather surprised that there is no Git-based Source provider. While I don't know how hard this would be to implement cross platform, it would most certainly be useful to be able to download sources by providing a Git address + commit id. While most services like Github offer the possibility to download files via http, a Git based provider could be useful for easily downloading more complex projects, e.g. those with submodules.