Updates to namespaces.py.
Updates to namespaces.py.

* Updated Namespace.__init__() to behave a bit more like the
  the one I was working from. It only calls
  __default_construct() if no *args are passed in.
* Added __lookup_uri() and __lookup_prefix() methods to
  NamespaceSet to reduce repeated code.
* A lot of style changes like flattening out if/elif/else
  blocks and changing up str.format() calls. This was
  mostly me just trying to figure out the code by
  messing with the trickier spots.
* Refactored get_xmlns_string() and get_schemaloc_string to
  be a bit quicker by reducing the amount of string concat
  calls (leveraging .join() instead).

CybOXProject/mixbox