Refactor layout of file and watch divs

Don't use the Bootstrap layout classes; specify the styles more directly

This will make it easier to implement a draggable border between the two
Refactor layout in preparation of making the watch pane resizable

Uses the technique of putting the smaller div in the margin of the
larger div: http://alistapart.com/article/holygrail
Put a transparent 3x div between the code and watch windows

We'll use this as the draggagle part for resizing
Allow the user to resize the code/watch divide

Inspired from
http://stackoverflow.com/a/6219522
and
http://jsfiddle.net/gaby/Bek9L/186/