[osx] Allow the user to configure a NSApplication subclass and use that instead.
[osx] Allow the user to configure a NSApplication subclass and use that instead.

Fixes http://hipbyte.myjetbrains.com/youtrack/issue/RM-539

katsuyoshi/RubyMotion,hboon/RubyMotion,hboon/RubyMotion,hboon/RubyMotion,incbee/RubyMotion,beni55/RubyMotion,jjaffeux/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,HipByte/RubyMotion,jjaffeux/RubyMotion,HipByte/RubyMotion,beni55/RubyMotion,katsuyoshi/RubyMotion,incbee/RubyMotion,jjaffeux/RubyMotion,beni55/RubyMotion,beni55/RubyMotion,incbee/RubyMotion,katsuyoshi/RubyMotion,katsuyoshi/RubyMotion,HipByte/RubyMotion