add build process for brownie.js library

This PR uses grunt to build a brownie.js file which contains all of the src stuff. This way you can just include brownie.js and not worry about other junk.  

`npm install` should be all you need to get the dependencies installed. After that `grunt` will build the brownie.js file and put it in the `build` directory. `grunt watch` will automatically rebuild brownie.js each time you make changes and save anything in the `src` directory.