add a demo project

Fixes #14 

![preview-gif-cloning](https://cloud.githubusercontent.com/assets/359239/4632476/e143d1bc-53bd-11e4-8327-5bf1b1855bc8.gif)

 - [x] implement a simple cloning demo
 - [x] add some extra context to the cloning messages to match up with How Git Does It
 - [ ] opt-out of checkout messages (not the progress count) after clone, it's just noise
 - [ ] after cloning is done, demo app should show options for checkout branch
 - [ ] implement simple checkout demo using branches found in repo

I'll punt on push/pull for this demo because they may require authentication (which is supported, but i'm lazy)