Don't try to checkout a commit for an ignored gom

When a gom is in a group that is not installed or for a different OS,
it is not installed. And so, gom should not try to checkout a specific
commit/tag/branch for it.
Fix gom install with sub-dependencies

For example, for this Gomfile:

    gom 'github.com/foo/bar'
    gom 'github.com/foo/baz', :commit => 'deadcafe'

When `gom install` is run, `github.com/foo/bar` was built with the
master from `github.com/foo/baz`, which can be not the specified commit.
Now, gom clones all the repositories and checkouts the specified commits
before building and installing them.
Add a note about gom install -u in README
Merge pull request #14 from nono/install

Gom install