Add condition to assign tracked host name in settings panel

We have a common problem where Parsely tracks our non-production environment URLs. We have had multiple instances where staging or non-production ready URLs pop up in our Parsely powered related links widgets. To prevent this, we are explicitly declaring the host name which Parsely should track.

We created an optional field in the Settings panel to declare the preferred hostname. If this is declared, then Parsely will not generate the <meta> tag to track the host name. 

![screen shot 2014-09-22 at 11 37 40 am](https://cloud.githubusercontent.com/assets/1332366/4359001/a2d59fd8-426e-11e4-890b-f81204ea54f7.png)