Improve/unify sti and gear help commands

@bparees review please.

Notes:
- `sti` now prints Long description while invoked without any parameter, the same way as `gear` does it
- `sti` and `gear` commands don't need Short description at all (it's never printed)
- unify `sti build` and `gear build` parameters (`build <source> <image> <tag>`) and it's description
- substitute `http://github.com/pmorie/go-sti` reference with `http://github.com/openshift/geard/tree/master/sti`