Prepare for buildbot automation

* The installation script was modified to recognize when its running on windows,
  as well as tweaking how it downloads and installs snapshots. The goal here was
  to make the script runnable on buildbot for mac/linux/windows with 32/64 bit
  options on mac/linux.

* The installation script now install rustc to `rustc/bin` in the local
  directory to have parallel builds on buildbot.

* The tests now store all their temporary state locally in the build directory
  to enable parallel builds on buildbot.

* A shell test is ignored which assumed the presence of a TTY output.