titleless window via window pref

Simple change to allow a title-less window (set in ```Preferences->Appearance```)
![screenshot 2014-02-24 22 12 59 copy](https://f.cloud.github.com/assets/868700/2255205/69715476-9df0-11e3-86cd-9ca66c03bd8b.png)



I also experimented with adding a title-less window style (```Preferences->Profile->Window->Style```), but I wasn't sure how to preserve ```windowType_``` in ```PseudoTerminal.m```.
Commit here: https://github.com/bmwang/iTerm2/commit/374bbd05358a713ac3c13ec8f2e5831221e686da . ```windowType_``` will eventually be set back to ```WINDOW_TYPE_NORMAL``` if you go to/from fullscreen. It works in this case because ```WINDOW_TYPE_NORMAL``` and ```WINDOW_TYPE_NORMAL_TITLELESS``` are identical except for the WindowMasks that go into them when they're created, but having ```windowType_``` out of sync seems messy.