Implementation of query transformation callback feature

Implementation of query transformation requested in ticket #66:
* Add query_transformation.h with internal types
* Add method session::set_query_transformation<T>() - assigns
user-defined function or functor
* Add session::get_query() - returns string with transformed query.
* Replace use of session::get_query_stream() with get_query()
* Keep session::get_query_stream() to access original query accumulated
in current session statement
* Add test_query_transformation to common tests.
Tested using VS2012 on Windows 8 against DB2, ODBC (PostgreSQL, SQL
Server), MySQL, Oracle, SQLite3.