Review, improve and document the static files handler

Changes include:

 *  Much simplified route configuration.

 *  Etag generation is now enabled by default.

 *  Web mimetypes are now detected by default. A bigger list of
    mimetypes can be detected without any additional library.

 *  Mimetypes can no longer be specified as a list. Copying this
    list for new connections is too costy. You can easily convert
    it into a function and pass that function to the handler instead.

 *  You can however specify a single hardcoded mimetype. Mostly
    useful when serving a single file, like an index.html file,
    to avoid extra operations.

 *  Specifying a path as a list of tokens is not possible anymore.
    Use either a binary or a string.

 *  Using a private directory will not work if the application
    was not started properly. Cowboy will not attempt to find
    the location of this directory if the VM doesn't know it,
    as this caused issues in some setups.

 *  Overall the code has been much simplified and clarified,
    and of course has now been documented.