issue #113 : CHORD is not implemented

Dear processing-js staffs,

I went ahead and implemented the issue https://github.com/processing-js/processing-js/issues/113
I followed the instruction in the README.md for the browserify and node minify commands. Then it generated some artifacts which are not related to the issue I think, but I included them in this pull request.
Otherwise I have implemented support for the CHORD, so that user could write something like
```
arc(50, 50, 80, 80, 0, PI+QUARTER_PI, CHORD);
```
* I have updated the existing javadoc(or doxygen) style comment. Someone need to update the reference page somehow to reflect this change.
* I tried to add a unit test but wasn't able to find a good way to deal with it.

Thanks
Jeonggyu