Append .py to runTests so that it could be installed.

As a result, we now are able to run tests against installed Portage.
Rename '__test__' to '__test__.py' to easily install it.
tests: fix running tests on installed copy of Portage.

This involved adding installation paths to configuration files
conditionally and letting repoman fetch metadata.dtd.
tests: fix running import & compile tests on installed copy.
Install Python modules using setup.py.
travis: test installing & running tests against installed copy.