Merge ompi and ompi-release repos

[jsquyres]

Github is finally allowing us per-branch ACLs, meaning that we can restrict who can push to which branches. Yay!

https://github.com/blog/2137-protected-branches-improvements

We'll want to test this capability, and then start talking through the logistics of merging the ompi and ompi-release repos. Here's a few issues I can think of:

1. All the outstanding PRs on ompi-release will need to be re-filed.
   • Might want to wait until after v2.0.0 is released (and possibly after all the pending v2.0.1 PRs have been merged...?).
2. Verify who exactly can click the "merge" buttons to release branches. We can probably simulate this by making different RM groups in ompi-release (v1.10 and v2.x) and test to see if the different RMs can't merge to the branches that they don't control.
3. Update all the wiki pages to remove the duality of ompi / ompi-release.

[jsquyres]

Done!