quick fix for arrow-end issues #471 and #506

This is a quick and dirty fix for the following arrow-end issues:

https://github.com/DmitryBaranovskiy/raphael/issues/506
https://github.com/DmitryBaranovskiy/raphael/issues/471

We simply make the `markerId` always unique. This seems to work in chrome and FF. 

A lot of re-factoring needs to be done to remove the housekeeping for marker re-use (like `markerCounter`), but that's not something I'll undertake.