Image clipping feature and performance improvements on Arm

Hello, 

I wrote two features to get better performance on SoC's like beagleboard:

EDIT2: I overlooked an issue, but it's repaired by commit 17. 


- Image clipping: Select region of interest to reduce copy and conversion operations.
 All data structures keep their sizes, but will partially updated only.

 Use cmake variable LIBFREENECT_OPT_CLIPPING to activate/deactivate feature.

 Example usage of clipping feature (cpp-Wrapper):

#ifdef LIBFREENECT_OPT_CLIPPING 
  int top = 100;
  int bottom = 100 ;
  int left = 100;
  int right = 100;
  setClipping(true, top, bottom, left, right);
  //deactivating with
  //setClipping(false,top,bottom,left,right);
#endif

 ToDo:
 Update other wrapper classes.

- Assembler code for ARM platform:
 The 11bit depth data are unpacked by convert_packed11_to_16bit.
 I replaced the function by an assembler version to get better
 performance on Arm platforms (tested on beagleboard xm, igep board).

 Use cmake variable ARM to activate/deactivate feature.