added ghuser and ghpass options; resolves #360

modified:   tasks/xt/install/index.js
- removed crud version resolver functions that are no longer used
- create clone url from ghuser and ghpass if set
- use checkout -f instead of reset --hard

modified:   tasks/xt/install/package.json
- version bump
Merge pull request #361 from tjwebb/isuses/360/github-credentials-cmdline

added ghuser and ghpass options; resolves #360