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)

Sorry, didn't address what you asked for entirely. The substring implementation is `indexA` to `indexB` (rather than `index` and `count characters`).

So between a choice of `itemUrl.length + 1` to bump the end index appropriately (after removing an index from the front with a `substring(1)`), or removing the first character alone and then matching on the rest, I went with choice #2.

If it's more clear to read `substring(1, itemUrl.length + 1)` I can switch to that. It appears an explanation in both cases may be justified so the reader doesn't have to question the choice.