Use Android RSA PNaCL module

- Use old slow implementation of Android RSA for Dartium (moved the dart implementation out of adb.dart)
- Use NaCL module for dart2js version
- Fixed dart wrapper
- More documentation in JS wrapper
- Fixed key generation on NaCL module and additional error management
- Fixed RSA public key format (will get "Always Allow ..." to work)

Fixed #1482

review: @devoncarew