Support Compojure style parameter validation

Add support for regular expression based validation of individual
parameters in URLs:

```clj
["/user/:id", :id #"[0-9]+"]
```

This provides a way to do some basic regex-based validation without
losing the ability to generate URLs from routes.
Better route generation errors

Add information about failing validations to route generation errors
Merge pull request #37 from travis/master

Support Compojure style parameter validation