Bring API more inline with QSql style classes.

Use internal ref-counting for pointers rather.  This reduces the burden
of memory management on the user of the library.

Add 'is open' guards. (defensive programming bad.. but if we don't do
this and it is used as a library mis-use becomes hard to identify)

Changed name of QUnqlite to QUnqliteDatabase.  QUnqlite is the overall
set of functionality.  The datbase object is subset of this, seen
clearly in existing documentation referencing the object from other
classes.

Documentation not updated yet (see you next week).

Roadmapping: plan on keeping direct header inclusion as this isn't a
'driver' style API yet.  however will add errorString() function and
very likely error object.  Also plan on adding JSON related classes if
built against Qt 5.