Adds reference to libgit2sharp
Fixes a runtime error
Fixes #29: Clones repo using libgit2.

No longer downloads and unzips a zip file.
Also cleans up the code in UploadController a little.
Better error message as per @JoshVarty's comment
Merge pull request #31 from AmadeusW/libgit2

Uses Libgit2sharp to clone repos. Fixes #29