Introduced support for UUID column type in PostgreSQL statement.cpp.
Unit test added for UUID column support testing all supported kinds of
UUID formatted strings on input and standardized UUID formatted strings
on output.
Introduced sqlite3_soci_error exception as subclass of soci_error.
This new exception exposes the SQLite3 result code returned when any
failure happens. Using this method is preferable to scrubbing string
error messages in soci_error exceptions. Useful when there needs to be
a distinction between general errors and database constraint errors.
Unit test added to prove the funtionality does what is expected.