WebGL extensions

Create initial framework for WebGL extensions.

Add implementation for getSupportedExtensions and getExtension.

Add the following extensions to start with - 
EXT_texture_filter_anisotropic
OES_texture_float
OES_texture_half_float
OES_standard_derivatives
OES_vertex_array_object

Any extension defined between @EJ_GL_EXTENSION_IMPLEMENTATION and @EJ_GL_EXTENSION_END is automatically added to the extension registry. Native extensions are automatically queried before exposing the actual extension. 

Test case : OES_vertext_array_object example - http://dl.dropbox.com/u/24635762/toji_vertex_array_object.zip