[Yt-dev] Activate / deactivate

[Matthew Turk]

Hi all,

Casey Stark added a new mechanism for activating and deactivating yt.
I've merged this in, modified only slightly to be more explicit in the
install script about which version is being loaded, and pushed this to
the main repo.  Casey, feel free to add any comments you might have on
this.  It also reduces the huge amount of output the install script
spits out.

If you have the chance to try out installing yt on a new machine, or
an old machine, it'd be great if you could check this functionality
out.  I think it could be very useful for developing as well as
keeping stacks of software separate.  No more manually loading and
unloading environment variables.

Best,

Matt