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 as described in https://github.com/jspm/jspm-cli/issues/78#issuecomment-52281274. Now jspm-git download function supports http://, https://, ssh://, .. and I did some testing with bitbucket and github and both worked for me. I haven't performed intensive testing yet but it seems to be stable.

Have fun with it :)

p.s.: make sure you've the latest beta release of jspm because there was a bug which caused custom endpoints to fail.