Fixed XML testing

Actually compare XML trees rather than relying on the text
representation being same as this is pretty much dependant on lxml
version.

Signed-off-by: Michal Čihař <michal@cihar.com>
Merge branch 'xml-testing' of https://github.com/nijel/osc2 into xml
- test.httptest: beautify (pep8 and one additional newline before osc specific imports)
- test.wc.test_package: test content-type (commitfilelist)

The content-type for commitfilelist should be "application/xml".
- wc.package: set content-type in _commit_filelist
- test.httptest: always use compare_xml if the content_type is application/xml
- added test.util.test_xml module

It contains testcases for the util.xml module.
- util.xml: added custom OscElement and OscStringElement classes

Their find/findall methods support an xpath expression.
These classes are used by default when parsing/constructing an xml.
- always use OscElement instead of objectify.ObjectifiedElement

(Renamed class "OscElement" in the remote module to "RemoteModelElement")
- pep8-ified code