Added alpha channel and Color functions

[mxgrey]

I've added an alpha channel for Shape colors. It turns out the whole OpenGL renderer pipeline was completely ready for an alpha channel, it just wasn't being used.

I also added some Color generating functions. Rather than create a Color class, I simply made a namespace called Color in dart/math/Helpers.h and put some functions in that namespace. I decided to inline all the functions so that end users can just check the header to see what actual color codes are being used for each of the color functions.

[mxgrey]

Also, I put a usage example in the softBodies app.

[jslee02]

I'd like to create actual Color class so we don't need to worry about the validness with color arithmetic operation. Also, Color class can be in common namespace.

[mxgrey]

I think being able to do normal vector arithmetic with colors is valuable. I'm not seeing the purpose behind a unique class for them.

[jslee02]

Color class will be able to normal vector arithmetic operations but it will clamp when the result is not valid. For example, result of (0, 0, 0) - (1, 0, 0) = (-1, 0, 0) is not valid so it will be clamped as (0, 0, 0).

[mxgrey]

I'm kind of worried that could be overbearing. At the end of the day, it seems like it's the user's responsibility to provide sane color values. I could imagine someone wanting to determine their colors programmatically, and having a color value that's temporarily negative (between arithmetic operations). But then if the color class suppresses that, it could mess up the computation.

[jslee02]

It's possible. Okay, let's merge this.

[jslee02]

There are conflicts with master. Could you merge master branch into this PR first?

[mxgrey]

Merged.