Update index.js

handle nested routers path correctly
added tests for nested routes. also fixed issue with trimming path
Trim path prefixes more like Express

Express trims `.use()` mount paths, so this updates to trimming paths in a
similar manner to: https://github.com/visionmedia/express/blob/4fe1073f105dbc3b8aa00adf33aa5a19f248117c/lib/router/index.js#L232-270