Make Tika an optional dependency in setup.py

- Add the extras_dependency group "Tika" for specifying that Tika
  support is desired at installation time.
- Update entry_points definitions so that any which require Tika support
  are specified as such.
Update tika import in etllib to allow for optional support.

- Tika dependency imports are now handled gracefully when ETLlib is
  installed without the optional Tika functionality. If cleanseBody (the
  only function at the moment which requires Tika) is called when Tika
  has not been installed then a relevant error message is printed and an
  exception is raised.
Remove INSTALL.txt reference from setup.py
Merge pull request #4 from MJJoyce/remove-install-file-refs

Remove INSTALL.txt reference from setup.py
Merge pull request #1 from MJJoyce/optional-tika-install

Make Tika dependencies optional