added architecture diagram

Addresses #18 

Put in a diagram of how the code architecture should work. Can update it if necessary! [Here it is on imgur](http://i.imgur.com/QcHpxAU.png) and I'll attach it here too. Hopefully this will be useful for people who are just starting to get a sense of how this works.

![calchart-viewer-diagram](https://cloud.githubusercontent.com/assets/1168853/3746040/1836e926-17b8-11e4-8e23-7694b9dab9e2.png)