Added virtual compute function

Function that can be overriden in child class features that computes the histogram feature on an image. Returns the histogram as a double array
Added public interface

Parameterizes the algorithm on the cell size, block size, number of histogram bins, and horizontal/vertical gradient operator
Fixed parameterization on bins

Removed hard coded number of bins (9) and replaced with the parameterized number that can be set using the classes public interface

Last three commits refine #4