Default environment variables and fixes for pick-update and pick-use

• Defaults are exported for $DISPLAY, $GPU_MAX_ALLOC_PERCENT, and
  $GPU_USE_SYNC_OBJECTS when they aren't present.

  Gabe Evans

• Fix pick-update to allow changing branches.

  Gabe Evans

• Fix pick-use to allow selection of miners with full version specifiers.

  Gabe Evans

Initial release

Initial release with support for version switching, installing/uninstalling (using miner-install), and executing miners.