Added a new exercise: "Recognizing Improper Fractions and Mixed Numbers"

![Exercise](http://i.imgur.com/dcwMs.png)

Although the exercise itself is a simple one (based on recognizing fractions 0.5), I had to extend the JavaScript a bit in order to draw the graphie elements at arbitrary locations (for the multiple pie charts, for example), so I extended the pieChart() helper to allow the chart to be drawn at any location (in a way that won't break existing code).

I also added similar versions of the questions for mixed numbers, where it fills the "whole" elements solid to hint the student towards the format of mixed numbers:

![Mixed numbers](http://i.imgur.com/vmfMF.png)

I added rectangle() function for convenience and code de-duplication because I didn't see one. However I had to put it in graphie-helpers.js and not graphie.js itself, as for some reason the exercise could locate it. I isolated the javascript for this exercise and commented everything thoroughly, so it should be good to go! Please let me know if I missed anything.

**[Sandcastle link](http://sandcastle.khanacademy.org/media/castles/phillco:phillco_fractions/exercises/recognizing_improper_fractions.html)**