Image Processing Chapter Edits

@golanlevin, here are my suggested edits for the first half of the Image Processing and Computer Vision chapter. It looks like @ofZach has gone ahead and removed your heading numbering system (which I was going to recommend), so I have preserved his changes. First off, great chapter! The "Pixels in Memory" and "Grayscale Pixels and Array Indices" sections are perhaps the best explanations of image data storage and representation that I have come across. Very informative and accessible, awesome job! 

I've made basic edits inline and used our editor convention of **[BD: commenting]** where suggestions are recommended but left to the author to change (or not change). One key edit that I have made is to change all occurrences of "testApp" to "ofApp" per the ofBook standard. I have also taken the initiative to add an `h1` title to your chapter with by lines to demonstrate the standard that we have been using (for the web + pdf versions). Feel free to change it as you please of course! 

It was a pleasure reading and editing this chapter. Let me know when the second half is finished and I will take a look at it.