Don't limit book depth results in LOP when using taxonomy.

#5774 

This PR makes the LOP display book pages with more than depth of 1 if taxonomy is used for filtering in the LOP widget.

Page for the term "Economy & development":
![selection_146](https://cloud.githubusercontent.com/assets/4497748/2898737/fef41b22-d5a5-11e3-81fd-2cc63bab8997.png)

Page with the LOP filtering by the term "Economy & development":
![selection_145](https://cloud.githubusercontent.com/assets/4497748/2898738/070fdac6-d5a6-11e3-865a-ddb0d7f0a103.png)