Getter that returns boost::optional

[sgielen]

There should be a getter to replace the following line:

boost::optional<uint64_t> foo(statement.is_null("foo") ? optional<uint64_t>() : statement.get_uint64("foo"))

With something like:

boost::optional<uint64_t> foo = statement.get_uint64_optional("foo)";

[NickNick]

This should be pretty easy now to create (check if is_null(), return boost::none or the value)