switch solr configs to solr 4

almost all tests passing, but spatial not working
[tox] run tests for 1.6, fix test modules so they are found by the new
test runner.

These changes are backwards-compatible with django 1.5.  As of this
commit, the only failing tests are the Solr extractraction test, and the
spatial tests.
fixed simple backend for django 1.6, _fields was removed.