overlay loops, matching std audio semantics

specific loop count in addition to infinite. matches the standard/common
looping semantics of many audio systems. doc'd overlay method with info on
the behavior of the loop & loops parameters (and rest while i was there)
loop param kept and meaning matched to keep backwards compatiblity.
switch to times rather than loops semantics, and adjust code accordingly
Merge pull request #76 from ross/looping

overlay loops, matching std audio semantics