add the WebGLReference annotation type
add guidelines section to README file
completely restructure Program class

The class lacked several integral features compared to raw WebGL. For
example, it is now possible to individually and repeatedly attach and
detach shaders.
annotate Program class

Add WebGLReference metadata to all members.
rename the WebGl namespace to WebGL

The [Dart style guide][1] proposes upper case style of two letter
abbreviations.

[1]:
https://www.dartlang.org/articles/style-guide/#do-capitalize-acronyms-and-abbreviations-longer-than-two-letters-like-words