[tddium] Support internally hosted TDDium projects

Support TDDium Enterprise, a privately deployable version of TDDium (https://www.solanolabs.com/#pricing) 

NB. TDDium has since rebranded to Solano, so I suspect that the tddium.com URLs that we have hardcoded may be deprecated. But since I don't use the one hosted on their servers, I'm really not sure.

![Allow custom instance URL](https://cloud.githubusercontent.com/assets/1108361/3787739/a148c682-1a31-11e4-89f6-8d401ea26edb.png)