added Konsole-like 24-bit color control sequences

Here's an implementation for 3-byte color mode: https://code.google.com/p/iterm2/issues/detail?id=218

I don't know Objective-C, but it works. My memory management might need some looking at, as I just [release] everywhere I can. 

It's fixed now, but there was also a minor off-by-one error in PTYTextView.m:_drawLine around line 6673 where the correct background color for the current character in a run was actually stored in the last one? I'm unfamiliar with the codebase, so I have no idea what was going on there.

Here's a quick screenshot of how it looks: http://i.imgur.com/TQArgxB.png

More details: https://github.com/robertknight/konsole/blob/master/user-doc/README.moreColors