Add edit commands to Firepad.Headless

[iphydf]

I've implemented autoformatting by calling astyle (a C code formatter) on a server and then setText()ing the result. An unfortunate side-effect is that everybody's cursors are reset to the end of the document, because the entire document is reloaded. It would be good if the server could instead supply small edits as if it were a regular user.

Bonus points: make this extra easy for me by turning the output from diff -u into an edit or sequence of edits via headless.