Open new tab if current one is busy

Something that's been bugging me is that when I run the tests if the window where the tests are to be run is busy (eg. tailing logs, vim session, server running, etc) current procedure will just output the rspec command into the busy window and I need to switch to it and and open a new tab. In this PR, I would like to propose that if the window is busy the plugin should open a new one.

So far, there is a problem. If a new window is open, it will reset the path, which isn't great. I was thinking of adapting run_in_os_x_terminal script to take in a parameter that would specify the current path as returned by :pwd in vim. Would do you think?