Fixed up a few minor issues with pull reqs 8 and 9.
added colorMiddle functionality; streamlined vertex shader code

* added colorMiddle property (enables three stages of color changing, e.g. red -> green -> blue)
* changed some attribute names for clarity and consistency, e.g. xxxStart corresponding to xxxEnd, and xxxStartSpread since it only affects the value of xxxStart; also changed customColorXxx to colorXxx for brevity
* in the shader code, removed Lerp functions and replaced by the pre-existing mix function (which has the same effect)
* simplified the cases in the "lerping" section of the vertex shader
Update ShaderParticleGroup.js
Merge pull request #10 from stemkoski/dev

added colorMiddle functionality; streamlined vertex shader code
Attempting universal gravity-based particles. GLSL version says no. Help
Removed stupid code from gravity.html
Fixed naming reference issues re mass -> particleMass. Obv. still same issue.
Figured out looping problem. See playground/gravityParticles.html
Updated gravity example
fix hidden particle values in vertex shader

code occasionally failed silently presumably because integers were being passed rather than floating point numbers (and changed the hidden particle color to black, since its hidden anyway)
Update ShaderParticleGroup.js
Merge pull request #12 from stemkoski/dev

minor shader fix
Merge branch 'dev' into dynamicAttributeUpdating

Conflicts:
	src/ShaderParticleEmitter.js
	src/ShaderParticleGroup.js
Merge branch 'dev' into dynamicAttributeUpdating

Conflicts:
	src/ShaderParticleGroup.js
Whitespace formatting.
Moved float/vector creation/randomization functions to seperate utils file to be more DRY.
Updated dynamic property setting.