[Reviewer: EM] Tel uris

Ellie,

This pull request contains the I-CSCF changes I've made for tel URIs. I thought it might be easier to review these separately to the rest of my changes, especially as they're finished. Let me know if you'd rather just review it all together. The rest of my work should be done by Friday morning. It'd be good to try and get this to Jim as soon as possible, so if you could review this and the rest of my changes by the end of the week that would be good.

Specifically, this pull request contains the following new bits of function:

*If we receive a terminating request to a SIP URI containung the user=phone parameter, we translate it to the equivalent tel URI.
*If we fail to find an S-CSCF on an LIR and we are terminating at a tel URI, we do an ENUM lookup and if we find a match we try the LIR again.

It's also worth noting that we had multiple versions of the is_user_global function, and it looked broken to me. I'd guess we didn't realise because we never had the enforce_global_only_lookups flag set. I've fixed it and moved it to PJUtils.

These changes have full coverage in the sprout UTs, and the full UT suite is running fine. That is the extent to whcih I intend to test these changes.

Thanks,
Graeme
It doesn't feel right that you have the function translate_sip_uri_to_tel_uri, and then follow that up by checking the uri produced is a tel uri - I think you should have these validity checks in the translation function instead. 
Also, should we definitely continue if the translation fails?