Mostly import cleanups, some doc enhancements.

Added fully-qualified namespaces to internal imports within encoders so they would link properly in the doc build.
Adding encoders to doxyfile.
Doccing arithmetic encoder.
Working on doccing base Encoder class.
Finished documenting base Encoder class.
Fixed typo in error message ('neither ... not' -> 'neither ... nor').
Mostly import cleanups, some doc enhancements.

Added fully-qualified namespaces to internal imports within encoders so they would link properly in the doc build.
Adding encoders to doxyfile.
Doccing arithmetic encoder.
Working on doccing base Encoder class.
Finished documenting base Encoder class.
Merge branch 'doc-encoders' of github.com:rhyolight/nupic into doc-encoders

Conflicts:
	py/nupic/encoders/logenc.py
Revert 8f1414a71378
Merge branch 'doc-encoders' of github.com:rhyolight/nupic into doc-encoders
Manual fix of crappy merge attempt
Method parameter name change.

`input` conflicts with python keyword.
Found more parameters named "input".

In base encoder.
Merge pull request #564 from csimons/typo

Fixed typo in error message ('neither ... not' -> 'neither ... nor').
Merge pull request #566 from rhyolight/doc-encoders-attempt-two

Doc encoders attempt two
Merge branch 'master' of https://github.com/numenta/nupic