improved SQL static shortcut handler

New session/endsession methods added and a database setter that accepts
\mjolnir\types\SQLDatabase objects has been included.

You can now do:

    \app\SQL::session($db);

    // the code

    \app\SQL::endsession();

If you need to work with database objects it's still recomended you simply
use the objects! however this is a nice shortcut and helps slightly lazy
code make use of a database object when one is provided (eg. migrations).
fixed bug in Pdx::insert method

The insert method will now correctly return the inserted entry; as
documented.
minor formatting fix