Test documentation examples in browsers

[robotlolita]

Currently documentation examples are only tested in Node. This is good enough to make sure that documentation examples are kept in sync with the code, which is very important, but there might be some browser quirks that aren't being caught in these examples, so an user might try them and get frustrated because things won't work. That's not nice.

Since the documentation files are loaded dynamically right now, we can't really package them with Browserify, thus it's necessary to add a new build step that figures out which documentation files have to be loaded and generates a static JavaScript file containing require calls to those. At that point, test/specs-src/documentation.js would just point to the generated file, rather than loading documentation tests dynamically. The browser-tests.js file can them just load that spec like any other.