Fixed PEP8 errors and made minor simplifications.
Fixed error causing the cached _schema to not be updated.

- This was causing conflicts between tests, causing some to fail
  by that.
Improved code of tests (PEP8 and repeated code).
Added TestCase to test against the actual terms in the database.
Added test confirming issue #1.
Fixed error on get_terms that caused the last term to end in '\n'.
Fixed #1 by making ID to be parsed as a non-text.
Added test validating issue #2.
Fixed #2 by making indexing of ID distinct from others.
Improved further the test for issue #2.
Improved backend interface and removed repeated loop.
Removed _meta calls by using the haystack 'get_model_ct'.
Removed minor unused code in tests.
Fixed error on test case.
Removed _meta calls by using the haystack 'get_model_ct'.
Simplified tests and added documentation.
Simplifed tests code by removing duplicate code.
Fixed error that was causing a test to fail.
Fixed error that was causing test to pass due to assert(True).
Simplified some conditions in the code.