Clear the terminal screen before running specs

* clear away the text/noise to help focus on the current test
* helps to see what test is being ran
Open new terminal window when current window is busy
Configurable iTerm support for MacVim

* Document runner scripts inside bin/
* g:rspec_runner is the command that runs rspec, defaulting to using
  bin/os_x_terminal
* Add bin/os_x_iterm to run specs in iterm