Adding iTerm support for Mac.

* It is probably a safe assumption that if an OS X user has iTerm installed, then that is their preferred terminal app over the default Terminal (Otherwise why bother having it?).
* 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.
* The if/else logic the picks the terminal could easily be modified to read in a terminal option from the settings screen. I just don't have time to implement right now.
cc @danthewolfe