add bootstrap option for views

For now, I've added ERB views only - no HAML yet. Resolves feature reqeust https://github.com/dhampik/rails-admin-scaffold/issues/6

There's still probably some improvements to make - but should be easy fixes from here.

Here's some screenshots of the bootstrap views:

![screen shot 2014-10-12 at 9 20 53 pm](https://cloud.githubusercontent.com/assets/145042/4605778/687785c2-51fa-11e4-8244-418ad67c3bb3.png)
![screen shot 2014-10-12 at 9 20 43 pm](https://cloud.githubusercontent.com/assets/145042/4605777/6875f036-51fa-11e4-9fc5-e62358efbdeb.png)
![screen shot 2014-10-12 at 9 20 38 pm](https://cloud.githubusercontent.com/assets/145042/4605779/687b1318-51fa-11e4-8392-237cb4cf1dc7.png)
![screen shot 2014-10-12 at 9 20 29 pm](https://cloud.githubusercontent.com/assets/145042/4605780/687d2b30-51fa-11e4-8463-8fa28d191a34.png)
![screen shot 2014-10-12 at 9 20 23 pm](https://cloud.githubusercontent.com/assets/145042/4605781/688098b0-51fa-11e4-9565-a19b0dd85abd.png)