Allow calling default methods
- Change bytecode version from V1_2 to V1_8.
- Enable stackmap frame calculations which is required for bytecode 
  versions >= V1_6. See [1] page 36.
- Enhancer#emitMethods(): detect default methods and emit an 
  INVOKESPECIAL instruction to the owning interface
- Omit calls to Object#inexistentMethods(), which was done at the end of
  each proxy method. This is not valid for newer bytecode versions
  anymore.

[1] http://download.forge.objectweb.org/asm/asm4-guide.pdf