Added an additional node function called getPixelColor().

Currently not very optimized, but seems fast enough for CLI work.  I'm still relatively new to V8 and Nan stuff, but it seems to perform somewhat well at least on my MBP.  Expecting comments here if you have any, since I'm unsure if the code is up to scratch for what you want.  I know there's a way to get the value from -convert, but I wanted it to be more speedy and not require information to go through `sed` to get it in a readable format.

I've been using in an internal script to detect pixel color/opacity slices in 9-patch images in order to re-draw them at different scales when resizing said 9-patch images for an Android app.  Since Android hates SVG graphics, many GUI widgets need to have different versions of an image for each screen "density"... and many of these items are [9-patch images](http://developer.android.com/tools/help/draw9patch.html), which rely on sharp 1px lines on a transparent 1px frame surrounding the image to show what parts of the image can be stretched when rendered on screen... the problem with resizing the images is that it resamples these lines rendering them useless at other sizes. tl;dr it's a huge pain in the ass for a lot of devs trying to automate the resizing of images for Android applications, and there's currently no good CLI script that handles this... so I'm writing one.

Aside from addressing my issues, there might also be a use for getting individual pixel colors for other people as well, since I don't think any of the nodejs implementations of IM seem to cater to that.

Anyway, just let me know what you think!  Thanks for checking this out!