refactor dialect to databaseType

QSqlDriverPrivate uses a set of enums in Qt5 to represent different
database types. This changes switches to using these values internally
in QDjango, in hopes of eventually switching to the ones provided by
Qt.
Merge pull request #23 from mbroadst/refactor-dialect-to-databasetype

refactor QDjangoDatabase::databaseDialect to databaseType