Added a modulate API method wrapper and tests.

I wanted to change the hue of an image using wand and found that in order to do so, I would have to use the low level API.  Rather than do that (and recreate the wheel) I decided to try and add support in the image module for wand directly.  It was relatively straightforward and I did a little hand functional test with the mona lisa PNG from wikipedia.

I decided to contribute this as I thought there might be other people out there who needed to use ImageMagick's modulate method.  So, I read the contributor docs and created a test case and docstrings for my new method.  If I need to do anything else to get this patch included, please let me know.  I tried to follow the project standards as closely as I could.

Thanks,

Dan