Updates branch issue123

This PR updates the branch issue123 according to the specifications in #123.

- 9907b45 refactors test code according to PEP 8 and removes duplicated asserts. It gives no semantic change to the existing code.

- 93522bb to 4d51f5e implements the correspondence explained in the [wiki page] (https://github.com/notanumber/xapian-haystack/wiki/Correspondence-between-Haystack-queries-and-Xapian-queries). They pass the `LiveXapianTestCase` (#123), and fail to pass on some tests from the other test cases (as expected, see #123).

- 5d16d1a adds support to Haystack AutoQuery, and d06da45 simplifies the code.