Rewrite to use KSCrash

KSCrash makes a much more solid base than the previous Bugsnag notifier, with support for on-device stack unwinding and C++ exceptions. This rewrite is now ready for limited beta testing.

* [x] Feature comparable to v3
* [x] Extensive CrashProbe testing
* [ ] Automated testing of Bugsnag code
* [ ] Mac OS X version

Known bugs:

* [ ] fails the malloc() test on CrashProbe intermittently
* [ ] occasional line-number off-by-one errors when symbolicated by the Bugsnag backend.