Fix bower installation issues, clarify usage

Could not install via `bower install angular-payments`. Syntax error in bower.json. 

![](http://media.tumblr.com/8e237c2f4d29ff1dab1b8ebc99860e93/tumblr_inline_mvmfttlwjq1s0m7nr.gif)

Also, Many projects need CC validation, but may be using other payment gateways. Clarified documentation to indicate that stripe wasn't absolutely necessary to use the other directives.