drush: added wrapper to use HOMEBREW_PREFIX.

Added a new bin wrapper script.

The purpose is to set the environment variables ETC_PREFIX and
SHARE_PREFIX - both pointing to HOMEBREW_PREFIX.

As an example drush locates site wide installed commands in
$SHARE_PREFIX/share/drush/commands. SHARE_PREFIX (and ETC_PREFIX)
defaults to /usr and hence drush will look for site wide installed
commands in /usr/share/drush/commands.

That location is outside of where Homebrew should write stuff so this
makes it impossible for other formulas to install site wide commands.

The new wrapper script sets ETC_PREFIX and
SHARE_PREFIX to point to HOMEBREW_PREFIX. And now it is possible to have
other formulas install site wide drush commands.