Refactored tests code.

Mostly PEP8, but also redundant asserts.
Changed name of variable from "field" to "field_name".
Reordered methods position in XapianSearchQuery.

Also removed two @staticmethod.
Added new argument on methods to build queries.

This allows more flexibility on constructing the queries.
For now the argument is not used.
Modified how the query construction methods are chosen.

- "content" is a special field with generic search,
but now allow filters.
- parameter field_type is now retrieved from the backend and
passed to the construction methods.
Refactored _term_query query constructor.

Its interface is explained in the docstring.
Refactored _phrase_query query constructor.

Its interface is explained in the docstring.
Refactored _filter_startswith query constructor.

Its interface is explained in the docstring.
Refactored _filter_exact query constructor.

Its interface is explained in the docstring.
Refactored _filter_in query constructor.
Added method to create OR queries from list of strings.
Fixed #98 - queries are now consistent with Haystack 2.X.

Refactored _filter_contains query constructor.
Fixed #101 - Adds support to AutoQuery.
Simplified construction of Xapian queries.