Move tests from tests/__init__ so unittest2's discover can find them

--HG--
rename : rst2rst/tests/__init__.py => rst2rst/tests/test_fixtures.py
Fix docutils 0.10 compat: error_reporting and math moved to docutils.utils
Merge pull request #18 from masklinn/master

Let's merge it now.
I created additional tickets to address some points later.