Added a modulate API method wrapper and tests. #134
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.