Added stopFollowingOnEvents and minor bugfixes involving the circle mark...

Resolves #43.

Added stopFollowingOnEvents and minor bugfixes involving the circle markers.

The interface works as discussed. When initializaing `L.Control.Locate`, provide an option `stopFollowingOnEvents` which is an array of strings representing event names. Example:

```
stopFollowingOnEvents: [
    'dragstart',
    'mousedown'
]
```

In addition, a regression involving the circle markers was fixed. A previous commit changed circle markers to use a persistent object, but made no provision for refreshing the style, thereby excluding cases where the circle styles are different depending on the following status.