Fix README link
(fix) Return 404 response, if file was not found

Instead of returning an Err and defaulting to a 500
response code when Static is unable to find a file,
we now correctly respond with a 404.

The 404 is returned directly from `handle` as opposed
to `catch` since it is not a Server error but a client
error.

Fixes #40
Merge branch 'hannobraun-bug/error-404'
Merge pull request #45 from EduardoBautista/fix-doc-link

Fix README link
(cleanup) Simplify tests file name
Add test for "redirect to trailing slash" behavior