Added additional method to create ad cells
Fixed cell addition to the array for dequeued cells

If cell was previously registered to be dequeued from the nib, method
will return already created ad cell and cell will never get added to
pool of known ad cells.
Added #ifndef for InMobi API key to allow external definition

With current implementation, it’s impossible to use the source
untouched. Adding test for existing define allows to set the key at
compile-time or otherwise externally.