Allow empty filters - "is not None"

[jrebiffe]

Corrects the issue #13 : changes the Get and Get Config keywords to avoid ignoring filter_criteria when filter_criteria doesnot contain any child. Additionally:

• Complies with PEP8, Programmer Recommandation, bullet 2 (line 1102) which states "Also, beware of writing if x when you really mean if x is not None"; and Google section 2.14.4 bullet 2 "Always use if foo is None: (or is not None) to check for a None value."
• Remove the FutureWarning raised by LXML, which appear for any filter. (only if use_lxml=True) due to bool method of the class Element of LXML.