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)

Ah the good ol' obfusciated code :(

```javascript
baseUrl = '/example/myPath';
// baseUrl.substring(0, 15) === '/example/myPath'
// baseUrl.substring(1) === 'example/myPath' - strip the leading slash
// baseUrl.substring(0, 15) === 'example/myPath'
````

In this case, `baseUrl` is the haystack, and `itemUrl` is the needle. That's why we substring only to `itemUrl's` length and match the smallest possible string.

The previous version would check if `itemUrl` was contained *anywhere* inside of `baseUrl` and would match middle and end of `baseUrl` causing too much to match. Another way to think about this is a `beginsWith` implementation.