Improve test support

• Use unittest.mock when available. This removes the mock dependency on Python 3.
• Tests no longer need to mock out pyramid_sqlalchemy.includeme; this is now handled by DatabaseTestCase and the py.test fixtures.
• Automatically make py.test fixtures available externally. This removes the need to copy & paste them over from the documentation.
• Fix error on pytest fixture example.
• Setup Travis to
  automatically run tests on CPython 2.6, CPython 2.7, CPython 3.3, CPython 3.4
  and PyPy.