Skip search/indexing tests if no indexers are available (second try)

Skip all tests in this module if none of lucene/PyLucene/xapian are available.  This should allow Travis builds to succeed (assuming other PRs for 2780, 2814, and 2829 are merged).

This does not attempt to clean up the logic or add Travis support for one or more indexers, both of which work efforts are covered by separate [bug 2819](http://bugs.locamotion.org/show_bug.cgi?id=2819) that I will do next week.

This is an updated version of [PR#35](https://github.com/translate/translate/pull/35).