Removed dependency on type long; field_type can now be datetime.
Fixed #56 - Implements single valued field facets using Xapian.

Replaces the brute force _do_field_facets with
Xapian faceting for single valued facets.

Xapian does not support multi valued facets yet; thus
the brute force is used in this case.
Fixed #109 - Raises InvalidIndexError when facet is not indexed.

Also added regression test.
Fixed #126 - Deprecates microsecond indexing.
Fixed #109 - Raises InvalidIndexError when facet is not indexed.

Also added regression test.
Changed variable name inside a function.
Under dev.
Changed backend column method to a property.
Improved how queries are constructed.
Fixed code error (tests were not passing).
Simplified code.
Fixed tests to account for the new query correspondence.