Implement bindings for debug output feature

This patch implements ARB_debug_output/KHR_debug. It's was an extension
before and now it's in core since OpenGL 4.3.

Interface is not totally match C version:
 - no userParam passed to glDebugMessageCallback();
 - glGetDebugMessageLog() imeplemented as GetNextDebugMessage();

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Merge pull request #137 from kiryl/master

Implement bindings for debug output feature
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Merge pull request #138 from go-gl/authors

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