Add request links functionality to the availability on the record view.

Closes #294 

There is a data-request-url attribute that is only populated if an item is requestable (meant to generate a request link from the live lookup).  If there is no `data-request-url` attribute then the item cannot be requested, otherwise it should get a request link if the item returns a due date.

#### Location request link
---
![location-request](https://cloud.githubusercontent.com/assets/96776/3365501/6f21600e-fb2f-11e3-89ce-00626ffe83f8.png)

#### Item request link
---
![item-level-request-links](https://cloud.githubusercontent.com/assets/96776/3365503/7946828a-fb2f-11e3-9b2b-7b4afab704c2.png)

#### Special SSRC request link
---
![ssrc-link](https://cloud.githubusercontent.com/assets/96776/3365505/83366404-fb2f-11e3-8d19-d78539612bf7.png)

https://github.com/sul-dlss/SearchWorks/blob/request-links/spec/lib/holdings/requestable_spec.rb#L23-L28