Fixes, enhancements, tidy ups

Hello, thanks for this great module. I've just used it on one of my clients sites and came across a couple of things.. 

I've fixed a bug as described here - http://japhr.blogspot.com.au/2012/10/ace-and-disappearing-text.html, where on saving a page with a codeedtiorfield in the cms, the edit form would reload with the codeeditorfield only showing the first few lines of code inside.

I've also added the ability to set the editor theme globally or per-instance, and tidied up some of the other  configuration code.

Cheers