Measuring text size

[LaylBongers]

This is a feature required to do common things such as centering text and implementing a text box. Perhaps like this:

let mut text = gfx_text::new(factory).build().unwrap();
let size: [i32; 2] = text.measure("Test Value");

[kvark]

:+1:

[Kagami]

@LaylConway please check measure function in 0.5.1. See #31 for details.