Fixes 'Find it' link errors for non-Green locations

Closes #733 

Moved `has_stackmap?` validation from Callnumber class to helper.

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![image](https://cloud.githubusercontent.com/assets/302258/3940082/b40c7cc0-24d4-11e4-8f38-d8e6458e13d9.png)
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![image](https://cloud.githubusercontent.com/assets/302258/3940081/a324faae-24d4-11e4-9db1-479d3965e42e.png)
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![image](https://cloud.githubusercontent.com/assets/302258/3940098/19d14ed2-24d5-11e4-8ce2-757e8d63839a.png)


Ah, I didn't catch that comment at the end of that ticket.  Totally makes sense now that I look at the result.

![3942974-results](https://cloud.githubusercontent.com/assets/96776/3984066/eee30cd4-2884-11e4-9f55-391c7d824782.png)

![3942974-record](https://cloud.githubusercontent.com/assets/96776/3984065/eee18b16-2884-11e4-9fd1-a14de614e278.png)

:+1: from me.