Add a test helper to compare XML strings regardless of etree implementation
Factor out hardcoded namespaces. Remain backwards compatible!
Add a textXMLAttribute method for retrieving attribute values easier.

testXMLValue(element.get("attribute_name"))
can now be replaced with the following in newly written code:
textXMLAttribute(element, "attribute_name")

This qrmoves the requirement to test if an element is None before using the
textXMLValue(val, attrib=True) method.  Will avoid errors like "None has no
method 'get'" in the future when querying unknown element/attribute
combos and the element is None.
Fix WaterML classes so they don't require a File object input
Merge pull request #59 from geopython/namespaces

Namespace abstraction and two helper methods to improve testing