New `ESSENTIA_PYTHON_NODOC` env variable for faster Python package
imports. Skip loading an instance of each algorithm to populate __doc__ and
__struct__ fields in its wrapper class, when this varaible equals to 'True',
'true' or '1'.

Instead of using an environmental variable ESSENTIA_PYTHON_NODOC,
prepare the metadata at the build stage. Load on import.

Use Python 3 equivalent for `iteritems` to avoid using Python 2 to 3
transitional package six.

Remove legacy code for Python 2.