Rename arm's git repository to nyx

Hi Sebastian, I've used the new name long enough that I think we can now safely switch over. Ran it by dozens of coworkers at work along with the tor community, with somewhere between a neutral to really positive response (on contrast with seth, which was pretty universally negative).

I'd appreciate the repositories being renamed as follows...

• arm.git => nyx.git
• atagar/arm.git => user/atagar/nyx.git

No rush in the least, so at your leisure.

Thanks! -Damian

PS. Not sure if we can rename the trac component at the same time. If you'd rather have a separate ticket for that then happy to oblige.