Add jsonp support for browserified tumblr.js module

This adds jsonp support to the browserify bundle for `GET` requests, and adds the missing `browser-request` module to the `package.json` dependencies.

To make the different dependencies work between server and client the files had to be broken up a bit, the public api is basically unchanged though.

`POST` requests will still not work on the browser due to cross-origin policies.
This is the only real change to the public-ish part of the api, but I couldn't find anyone using `require('tumblr').Client` directly that I could find. It may warrant a larger version bump to make sure any of those edge cases don't break when they update.