Added NPCs

NPCs spawn at semi-random intervals and at random x-displacement along
the shoreline and scroll down the screen at 'scrollRate' pixels per
frame.  'scrollRate' can be modified by pressing 'KEY1' from 0 to 10
pixels per frame.  The PC scrolls up the screen at 1 pixel per frame and
can be moved left and right within the central section by pressing
'KEY3' and 'KEY2'.

Keyboard remains untested and SD Card has a bug in which the first
letter of strings gathered from the SD card is repeated 3 times in the
character array in which it is stored.