Add finger sliding

[mattvchandler]

Idea from @a-racoon in #2.

Allow for finger sliding in addition to swiping. When you hold your finger down, a new ball will appear every 0.25 or 2.0 seconds (depending on pressure). The direction you last slid your finger is the direction of gravity. You can do two modes; direction of finger from screen center, or relative from last drag (of 5 or 10 pixels).