Improve font rendering and outlines

To remove jaggedness of outlines, we now set context.lineJoin to `round`. After doing this, I realized that lots of letters where still being cut off on both sides.  It turns out the bounding width of the letter that we compute is actually wider than the browser computed width.  Using our own values prevented the cutoff.

I used thie jsFiddle to compare our per-letter rendering to typical "whole" word rendering in browsers: http://jsfiddle.net/AbdiasSoftware/48m4B/2/ We should match the bad kerning and all.

Fonts also look better at smaller resolutions now, so I removed the label hack I had in simple.czml.

I still want to test this more with a variety of fonts and settings to make sure it's good, any help in that area is appreciated. @shunter, should I role my changes into the ThirdParty measuretext?  We can't modify `width` directly because it's readonly, which is why I created computedWidth.

Fixes #1141

![image](https://cloud.githubusercontent.com/assets/1495196/5417922/21bf2026-820c-11e4-96e1-df41528fe23e.png)

CC @emackey