A couple of tiny PEP8 fixes
Improve URL var setup

There is one big, back imcompatible change: URL vars are now delimited
by `<...>` instead of `{...}`. This is because angle brackets are
unlikely to occur in the regular expressions for URL vars while curly
brackets are quite likely to. The upshot is that we can simplify the
URL var regex quite a bit by using `<...>`.

In addition, a converter can now be specified like so:

    /<(int)id:\d+>

When a URL matches a mounted resource, URL vars will be converted
automatically, and when generating URLs for a mounted resource, the vars
passed in will be validated by ensuring they can be converted (although
perhaps this is too restrictive?).

Started adding tests for MountedResource, especially relating to these
changes.
Add docstring for Application.mount_resource
Fix static file handler bug introduced in 4e7b35d

When calling into the DirectoryApp, we need to pass a request with a
proper environ, not a blank request.
Add "add slash" functionality for mounted resources

If a resource path endswith / or if add_slash is explictly enabled,
requests to the path without a slash will be redirected to the path with
a slash appended.