Improve gom install for existing sources

`gom install -u` is not a good idea:
- it fails because go install doesn't accept the -u flag
- it forces to update all the repository
- it's more complicated for the users.

So, I've made a change in gom: it now uses the network to update the
repository if it can't find the correct commit/branch/tag.
Merge pull request #16 from nono/reinstall

Improve gom install for existing sources