Buffer refactor

This is a massive refactor, which brings with it several nifty features:
- better support for CRLF (but we're not there yet);
- speed (block operations are significantly faster);
- a few new features, and proper support for D in visual mode
- support for XCode 5.1
- native undo support
- better layering : XVimBuffer, XVimView and hopefully eventually better testability.

It has stability improvements all over the board, fixes for end-of-line motions, ...