New file name for the codec, and name space added

Hi Junichi,

I modified the file name in OpenIGTLink library. 
So VP9Encoder.h and VP9Decoder.h will be igtlVP9Encoder.h and igtlVP9Decoder.h.  Also the code classes are in the "igtl" name space, so you might need to add igtl:: in you code.

Best,
Longquan