UI fixes.

This addresses two issues mentioned on #7: resizing the code area and changing how errors are displayed. These were addressed as follows:

* **Resizing the code area** - The CodeMirror API does not support a resize handle for the syntax-highlighted text area so I have added a custom handle that allows the user to vertically resize the code area, while looking as close as possible to a normal resize handle. This has been tested in Opera, Chrome, Firefox, and Safari so hopefully works across the board.
* **Error display** - Errors are now displayed as soon as the compilation fails, but instead of underlining, the errors are highlighted in a light red colour. When an error is hovered over in the error box, the highlighted code will become a darker red.

I believe that these changes work well, but there may be some more edge cases in there somewhere that I missed.