Added getBufferSubData and removed enums related to MapBufferData.

Also updated last modified date and regenerated WebGL 2 IDL.

Addresses Issue #541.
Reformatted paragraph.
Merge pull request #598 from kenrussell/get-buffer-sub-data

Added getBufferSubData and removed enums related to MapBufferData.
Removed UNIFORM_BLOCK_NAME_LENGTH enum, not needed in WebGL
Merge pull request #601 from toji/block_name_length

Removed UNIFORM_BLOCK_NAME_LENGTH enum, not needed in WebGL