MANIFEST.in: Distribute the tests/ directory

Avoid:

  $ python setup.py test
  ...
  warning: no files found matching '*.py' under directory 'tests'
  ...
  ----------------------------------------------------------------------
  Ran 0 tests in 0.003s

  OK

by packaging the referenced files.
Merge pull request #71 from wking/distribute-tests

MANIFEST.in: Distribute the tests/ directory