[qtcontacts-sqlite] Make ContactsDatabase a proper class
[qtcontacts-sqlite] Move properties into database class
[qtcontacts-sqlite] Keep SQL statements local with the code that executes them

Cache prepared statements by hash of their text, in order that
statements can be prepared at first use rather than as pre-created
member variable.  This allows us to keep the text of the SQL together
with the code that populates the bindings.
Merge pull request #140 from matthewvogt/localize-prepared-statements

Localize prepared statements