Use Ace editor to highlight code tabs.

This commit changes the code tabs to use the [Ace Editor](ace.c9.io) for syntax highlighting.  This also allows us to create editable code blocks.  All Ace code blocks, whether editable or read-only, support a number of nice features, including Command-A to select all, Command-F for searching, folding of code blocks, matching of parens, etc.

/cc @jegonzal