Make Bugsnag categories public

I removed the `public_header_files` option from the Podspec, which was making only `Bugsnag.h` public. The use case for me was that I wanted to test that `UIViewController+BSVisibility` was finding the correct view controller. I imagine that any of the categories, like the `NSNumber` filesize category, could potentially be useful.

I maintained the other 'main' Bugsnag classes being private. Not sure if there's a shorter approach to writing that in the podspec.

Additionally, I deleted comments leftover from `pod spec create` from the podspec.