Add base user agent string for self manager subclasses

[kltm]

While we can currently use hacks such as certain patterns (e.g. jQuery jsonp in some golr manager uses) to determine whether something may be originating from a particular manager library, this state of affairs is hacky and silly.

This ticket is to add a base for user agent strings for managers to work off of.

Also see:
biolink/ontobio#190
biolink/ontobio#191