Fixed Issue #1:
      TypeError: get() takes 1 positional argument but 2 were given #1

Originally had meant to set the method `get` on the `NavigableDoc` object
equal to  `self.collectiondoc.get`. This should resolve the issue in a simple way.
Updated documentation and version number to reflect fix to Issue #1:  method parameters.
Completed tests for NavigableDoc module.

Updated pmp_api package so that NavigableDoc and Client could be
imported directly. Updated README.md with examples to reflect this.