Finishing network support for libgit2

by Michael Schubert for Git

So far, libgit2 supports fetching objects over http://, git:// and file://. This year's GSoC task is to finish network support for libgit2, namely * fetch over ssh (including an API for SSH key management), * support for packing objects (indispensable for push), * push over http, git and ssh. The existing network code is written in a modular manner and allows to be reused (regarding socket handling for example).