build with macosx 10.10 and xcode 6.1

* re-introduce Python.framework into xcode project
* replace "-DLEOPARD" with "-DMACOSX", so we can tell if we are using
  the Python.framework or using the UNIX way.
* add a helper method to access SunPinyinApplicationDelegate from NSApp,
  otherwise clang complains.
macosx: use ARC
macosx: remove unnecessary checks for null