Add looots of comments

Summary:
Add comments everywhere! Also fix some small bugs like using Style.id
instead of Style.size, and rename some variables to be more descriptive.

Fixes #22

Test Plan:
 - Make sure the huxley screenshots didn't change
 - Make sure the tests still pass

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13158

katex.js

@@ -1,9 +1,21 @@
+/**
+ * This is the main entry point for KaTeX. Here, we expose functions for
+ * rendering expressions either to DOM nodes or to markup strings.
+ *
+ * We also expose the ParseError class to check if errors thrown from KaTeX are
+ * errors in the expression, or errors in javascript handling.
+ */
+
 var ParseError = require("./ParseError");
 
 var buildTree = require("./buildTree");
 var parseTree = require("./parseTree");
 var utils = require("./utils");
 
+/**
+ * Parse and build an expression, and place that expression in the DOM node
+ * given.
+ */
 var process = function(toParse, baseNode) {
     utils.clearNode(baseNode);
 
@@ -13,6 +25,9 @@ var process = function(toParse, baseNode) {
     baseNode.appendChild(node);
 };
 
+/**
+ * Parse and build an expression, and return the markup for that.
+ */
 var renderToString = function(toParse) {
     var tree = parseTree(toParse);
     return buildTree(tree).toMarkup();