Fix class documentation of CircleFlowIndicator

In HTML, a list item should be enclosed in <li>, not <ul>,
which is for the whole list.
Add snap option to CircleFlowIndicator

like ViewPagerIndicator's. It makes the active indicator jump to the
next position instead of following the scroll offsets. I've also
documentated it in the README.
Merge pull request #94 from orac/circleflow-snap

Circleflow snap