Merge pull request #127 from jorgecarleitao/issue123

Fixed #123 - improves Xapian usability with Haystack.
Applied 2to3 to convert all code to Python 3.X.

- Related to #128, but doesn't fix it since there
  are no tests for Python 3.X
- It passes all tests in Python2.7
Improved how date and datetime is indexed.

They can now be text-searched using UTC time format, on date, time
or both.
Added tests to check that every field type is correctly indexed.
Fixed #119 - Adds support to non-anscii indexing and search.
Refactored _query_from_search_node into two menthods.
Added test for multivalue indexing.
Reorganized, renamed, and improved docstrings of TestCases.
Improved robustness of auto_query test.
Improved robustness of test.
Added regression test for #103.
Fixed #103 - improves support for exact search.