BREAKING CHANGE: Major rework of the download function

Replaced the download function against a simple git clone which just
clones a single branch without history. Now the download function
supports http://, https://, ssh://, ..
Fixed dependencies
Make jspm-git compatibel to jspm 0.7.x
Merge pull request #1 from subesokun/feat-git-download

BREAKING CHANGE: Major rework of the download function