Finished ngin.core doc-comments

Finished adding doc-comments to the classes in the ngin.core package
(Only exception would be ngin.core.resources.ThreadedLoader).
Began commenting mixin classes

Started adding doc-comments to ngin.mixins.* classes.
Finished comments

Finished adding comments to all the classes. Also removed the
ngin.mixins.visual.bounded module, as it was obvious the module was not
truly needed or useful. Also started work on modifying the
ngin.core.container to add a Angle class.