Updates to entities.py module.
Updates to entities.py module.

* Some code-style changes: guard clauses used to reduce if/elif/else
  block nesting in loops; kwargs used when passing in literals to
  multi-parameter functions; use parens for multi-line conditions
  rather than backslashes; etc.
* Removed  Entity._get_children() and Entity._get_namespaces()
  as they are no longer used outside of two python-cybox tests
  and are no longer a part of the namespace collection/generation
  workflow.

CybOXProject/mixbox