created bin/.path/ directory the public bin dir

the public binaries that are to be on user shell should be added to
bin/.path/ as links. The actual bin/ directory has too many commands and
most of it are not useful to the end user. So having bin/path/ for user
commands is handy, specially for external packages installed, e.g. drush
(that is the actual reason that this file is being modified right now).
changed bin/drush helper to fallback to system first

bin/drush helper used to fallback to devpanel drush version, and then to
$PATH (if any). This commit inverts this logic (the subtle !hash drush does
the trick).

Also added some variables for it to be easier to read and
understand what's happening.

Also moved bin/drush to bin/.path/drush to take advantage of the
profile.d/devpanel.sh path set from previous commit.