Do not export addModule as part of the global API.

This pull request gets rid of `addModule` as part of the public webfontloader API. The reasons for removal are:

* It isn't documented.
* It doesn't work. The module API was never exported so all module functions called by the webfontloader core were renamed by the compiler which dynamically added modules would never know about.
* It is slightly smaller in file size: 18187 bytes (6747 bytes gzipped) before, versus 17384 bytes (6493 bytes gzipped) after.

This also means the `BrowserInfo` and `UserAgent` APIs are now private since they were only used by `addModule`.