allow customization of prev/next icons

For those who do not use bootstrap's glyphicons, and would prefer to select a custom icon for the previous and next arrows, I added two attributes to the configuration object to do this (previousIconClass and nextIconClass). The values of these configuration attributes can be any valid input to the ngClass directive (string, array, object).

Let me know if you think this change belongs in the project, and if you'd like me to adjust anything. I have also included a few unit tests to cover the feature.