Only load the google client.js script if gapi is undefined

[jacobcsmith]

We found that incorporating this module significantly slowed down our Angular 1.5 karma-jasmine tests. Since the run block of angular modules are automatically executed for every test we found that the google client.js was repeatedly inserted and loaded and this slowed down the tests. This duplicate client loading could be avoided if the angular-google-plus run function would check to see if gapi was already loaded before proceeding with the google client script insertion.