fix(menu): Update active menu determination logic

This fixes too rxMenuItem matching too much if multiple items have a url containing, for example, `/accounts`.

![image](https://cloud.githubusercontent.com/assets/5025474/3881857/6699530a-2191-11e4-9e2c-fb15c94e6f85.png)

While I do like the mocha reporter, is this to be a default in encore-ui?