A bug fixed in click event of pies

[HRezaei]

Fixed an error spotted in the event of click on segments.
In almost all (not checked all) samples, pie is defined as a global variable and is used as ID of an element in the page, thus made the error invisible.
To reproduce the error you can change simple.html and rename pie variable (line 24) and ID of div element (line 18) to something else, then open the page and click on segments.

Fixes issues: #174 , #144 , #133

[benkeen]

Hi @HRezaei, thanks for the PR!

I actually haven't been supporting this library for some time and rather hoped someone would be willing to take the project over. I posted a comment at the top of the README hoping for that, but no dice!

As mentioned, I'm heading out of town for 2 weeks and won't be doing much dev work, but let me revisit this lib when I get back and see if I can get a few things updated without spending too much time on it.