Shrink, repackage, and simplify

This shrinks the library, repackages files for browser use, and adds a separate path to including walverine.

* `citation.js` doesn't include walverine - instead, `citation-with-walverine.js` will get you Citation + walverine.
* `browser/citation.js` and companions also don't include walverine.
* The version of walverine that you'll get is now `0.7`, after @adelevie merged https://github.com/adelevie/walverine/pull/11, which removes its own downstream underscore dependency, lightening the load for everyone.
* There are now 3 more files in `browser/` - `citation-with-walverine.js`, and its two `.min.js` and `.min.js.gz` companions. These are what used to be `citation.js` and companions. The gulpfile has been updated with tasks that output all 6.
* The handwritten boilerplate at the top of every included file besides `citation.js` is gone, in favor of a straight `modules.export`. `browserify` handles making that work in-browser on its own.
* Similarly, some of the require-ing boilerplate from `citation.js` was cut out and simplified. A `window.Citation = Citation` is left, so that browser includes of `citation.js` get Citation added automatically to the window namespace.
* In Node, there is now 0 leaking into the global namespace -- not of underscore, XRegExp, or Citation.

Thanks to @adelevie for all the help and nudges to get this done. By default, citation.js.min.gz is now a mere **2,748 bytes**. Feeling real good about this.