Refactor native part

[EyalAr]

In order to facilitate the transition to NAN (issue #28) and allow more flexibility in image sources other than files on disk (e.g. network, streams) (issue #23); the native module will be broken down to 3 modules:

1. Decoder - Will receive a Buffer object of an encoded image (JPEG, PNG, GIF, etc.) and will create a buffer of uncompressed pixels data.

   // buffer is a jpeg image loaded from disk (e.g.)
   Decoder.decode(buffer, 'jpeg', function(err, pixbuf){
       // pixbuf is a buffer of RGB pixels.
   });
   

2. Encoder - The counterpart of 'decoder'. Will receive a Buffer object of uncompressed pixels and will create a buffer of an encoded image.

   Encoder.encode(pixbuf, 'jpeg', function(err, jpegbuf){
       // jpegbuf is a jpeg image
   });
   

3. Image - Will receive a buffer of uncompressed pixels and create an 'image' object, upon which image operations can be made.

   Image.create(pixbuf, function(err, image){
       // image.resize(...)
   });