Only attempt to handle GET and HEAD requests

The way in which this middleware is attempting to handle requests by
sending "blind" redirect responses only makes sense when the original
request is either a GET or HEAD.
Automatically determine whether to test with or without a trailing slash

This changes the behavior of this module to be smarter about whether it
should test the new URL path with or without a trailing slash. Since
this middleware should come after `app.router`, we already know that the
URL path as is does not have a matching route. If that original path
contains a slash, then it should be remove and visa versa. This new URL
path can then be checked against the app's routes to determine if there
is a match.

Closes #1
Merge pull request #2 from ericf/auto

Automatically determine whether to test with or without a trailing slash