Attempting to make gvm a first-class citizen

with usage sticking pretty closely to how rvm works, for better or
worse.  There are some issues with how gvm's install command works, plus
invoking `gvm use` overwrites the GOPATH such that re-setting from
within the `setup` method is needed.  It's a step.
Folding the GVM module into the Go module per feedback
Removing now-unnecessary `|| true` from `gvm install`

now that upstream is fixed.  Also added a comment about why we're
prepending to $GOPATH.  All per feedback from @joshk.
Merge pull request #110 from modcloth-labs/gvm-citizenship

Attempting to make gvm a first-class citizen