adds 'BW::NetworkIndicator'

Regarding #263 and #171, I agree w/ @clayallsopp that `UIApplication.sharedApplication.networkActivityIndicatorVisible` should be controlled by a centralized module.

I didn't call it `BW::HTTP::NetworkIndicator`, because I think it could be used as a standalone module (maybe they're not using BW::HTTP).

methods available:
- `BW::NetworkIndicator.show`
- `BW::NetworkIndicator.hide`
- `BW::NetworkIndicator.visible?`
- `BW::NetworkIndicator.reset!`

Specs included.