Add a seperate rust-try repo and corresponding automation

[brson]

We've about outgrown the phase where I'm comfortable with people just pushing to the main repo to do test runs, and try pushes are one of the main reasons we have to give people write access via the 'Rust-Push' group. Let's move this functionality to it's own repo, and possibly even to it's own automation infrastructure.

[nikomatsakis]

On Tue, Jun 17, 2014 at 11:55:40AM -0700, Brian Anderson wrote:

We've about outgrown the phase where I'm comfortable with people
just pushing to the main repo to do test runs, and try pushes are
one of the main reasons we have to give people write access via the
'Rust-Push' group. Let's move this functionality to it's own repo,
and possibly even to it's own automation infrastructure.

It would also be useful to have try runs test a superset of what bors
tests -- or at least have the option to do so.

[steveklabnik]

Becuase of the way GitHub works, we can't really remove push permissions. As such I'm giving this a close, sad as it is.

(it'd be awesome if only @bors had push ability)