Fixing beatloop start position, when beat length<1

I have been noticed: when I press beatloop button, it always starts loop in the start of the beat (if quantize is enabled of course).

When length of beat is >=1 it is ok, but when I need beat len<1 it is not good behavior.

EXAMPLE
(```|``` - beats, ```^``` - current track's position):
```
...|...................^........|...
```

When I press 1/2 beatloop, it create loop from 0% to 50%, but I want from 50% to 100%
If I press 1/4 beatloop, I want loop from 50% to 75% etc

This patch is fix the problem.

It is also possible to make cool effect with slip_mode+beatloops now (add some echo, double beats etc, or even extra short loops like here: http://www.youtube.com/watch?feature=player_detailpage&v=_qiYi8H2nDU&t=108 on 1:48 - 2:05)