Pressing enter on server password input joins the server

For these changes, I've added an extension for AngularJS which adds much needed keypress directives. It's called [AngularUI](http://angular-ui.github.io/); I created a custom build of it which only includes the keypress module, so there isn't much overhead.

Implementing the keypress event was as simple as adding the uiKeypress directive to the input DOM element. It's a really nice library and saved me the trouble of having to implement a keypress directive myself.

The original request for this feature was posted here: http://facepunch.com/showthread.php?t=1251011