make mistakes a requireable module, use browservefy for local dev

this has a number of changes:

- fixes a discrepancy where your package.json specified the nonexistant `index.js` as the main: moved contents of site.js into index.js
- changes example page to only require a single compiled bundle instead of multiple bundles
- wraps the JS syntax for codemirror in a module.exports function so it can be require()'d
- moved inline script tag code from demo page into site.js
- implements `npm start` using `browservefy`, which builds the bundle each time you refresh the page
- installs local copies of browserify and browservefy to make setup easier (people wont need to have globally installed them)

I think thats it