Refactor for nicer test setup.

``python setup.py test`` works out of the box.
Added tox.ini.
Add travis config.
We are compatible with Pyramid 1.3 and up.
Add tests which show that multiple skins selected by request type work.
Make pyramid_zcml optional and test/fix skin reloading.
Bump version.
Make zope.component optional, it's only needed when using an older zope.interface release which doesn't have ComponentLookupError yet.
Merge pull request #5 from fschulze/make_zcml_optional

Make zcml optional
Log missing skin path in watcher thread.
Prepare pyramid_skins 1.2.