Adding iTerm support for Mac.

* Modified applescript that is used to launch Terminal to check if iTerm
  is installed.
* If iTerm is installed, the terminal button defaults to opening iTerm.
* If iTerm is not installed it performs the old behavior of opening the
  default terminal app.
Merge remote-tracking branch 'danthewolfe/iTerm-support'

Conflicts:
	shell/terminal.osa
Changelog updates...