do_prjresults: support --xml option

"prjresults --xml..." simply prints the raw xml which it receives
from the api.
- tests.test_difffiles: removed "count" argument from re.sub in __canonise_diff

Old python versions don't support "count". This should fix the jenkins build
again.
- make travis-ci happy again...

It got broken in commit 6a332bc1c651b1555a5891f0a7db07795f4602ca
- sigh... make jenkins happy again (finally)

The jenkins build got broken by commit 178120c99e0b2fa7c158f589b4781c4b772fed45
(python 2.6's re.sub doesn't support a flags argument).
- refactor assertEqualMultiline into common.py for reuse

This will be reused by the tests for the imminent prdiff subcommand.
- add prdiff subcommand and test suite