Bash: source external config after brew init

We now source the external shell alias and shell command files after
initializing 3rd party tools like homebrew and initializing rbenv, etc.
This allows code in those files to test if required 3rd party programs
are installed before executing.
Bash: remove xterm check before modifying $PS1

The check was already wrapped in a check for an interactive mode
terminal. Since it's safe to say we'll always be using an xterm
compatible terminal if we're in interactive  mode, remove the check.
Bash: add command to display OS X notifications

We now have the notify, notify-success and notify-failure commands. The
latter two will display a OS X Notification Center notification and
play either a success sound, or a failure sound. The former will run
the command passed to it, then display a success or failure
notification based off the exit code of the command.

These commands rely on the program `terminal-notifier`, which can be
installed via Ruby Gems (`gem install terminal-notifier`). If this
command is not present, both commands will just use the `say` command
to speak the passed string using Mac OS X's speech synthesis engine.