Directory API should allow us to query by source id

[pgulley]

As title says- there is no way to query a source just by ID in the API. Should be an easy thing to fix.
Evan, just tagging you here in case you get around to it first- this is relatively low priority.

[pgulley]

For that matter, there's no way to do the same for collections either- ie, if I know a collection ID, I have to query for the whole collections list and filter it down! I think this is really quite straightforward, I'll move on trying to get this going asap.

[rahulbot]

Oops! I think I wrote that code on the web server using Django ViewSets a long time ago. Perhaps I just missed a method the REST endpoint method to return by ID?

Once in place, that'd be a good thing to add to the Directory sample notebook too.

[pgulley]

Addressed in #95!
I'll leave this open for the documentation task.