[tlu] beta with texdist switching

[Adam R. Maxwell]

I've just released a TLU beta with support for switching
MacTeX distributions. This is _very_ early, and doesn't
have a lot of error checking yet; if it breaks something,
let me know. 

*** YOU HAVE BEEN WARNED ***

I've tested it against TL 2015/2014{basic} and it works
okay on Yosemite. I'm not sure how it'll handle other
architectures; at present, I don't allow changing that
level of granularity.

https://github.com/amaxwell/tlutility/releases/

There are plans for improving this and making it better,
so it can actually repair things when /usr/texbin goes
missing, but that's not in place yet.

thanks,
Adam