Simplified updating interface and caching improvements

[mikolalysenko]

This change set replaces .updateExports() with a simpler method called .update(). This new method takes a new vertex/fragment shader runs them.

If there is a down vote on this, then I will merge this change set into gl-shader.

Longer term, I think that maybe glslify should move to sourceOnly as the default and only option, then the particular choice of shader wrapper wouldn't matter as much.

[mikolalysenko]

Summary of changes:

• Change handle to program
• Implement caching and reference counting for shader objects (improves performance when attribute locations are changed)
• Remove .updateExports()
• Add a new .update() method which reruns the constructor (similar to behavior from gl-buffer, gl-fbo, etc.)
• Fix a bug with attribute locations not working correctly
• Make default attribute locations deterministic across different WebGL implementations
• Add an option for using runtime reflection to load uniforms/attributes if not specified by glslify or otherwise

[hughsk]

Looks good to me! :)