progressive-loading-api: Progressively return API result while built

First draft implementation, for experimentation.

Add `&progressive=1` at the end of an existing API query, and it will
return the results progressively as they become available, instead of
everything at once at the end.

There are three parts on which I'd specifically like to have comments
before finalizing this:

* The `PROGRESSIVE_RESPONSE_BEGIN` tag string is currently very long,
  as in Firefox I only see the content being displayed
  progressively after a certain amount of data has been sent by the
  server - it is thus voluntarily long to allow to see the data
  displayed live in the browser while testing. We might be able to get
  rid of this tag completely if the readyState is immediately fired by
  browsers (ie if this is just behavior when *displaying* the data in
  the browser).

* Is the format of the separator good for you? We can change those, I
  don't particularly like the current format : )

* Currently the whole API response is resent every time - this allows
  to make things more generic, and even allow to modify information
  already sent in previous updates. Size is likely not an issue here,
  but if it makes things unpractical on the client side let me know.

Another aspect to be careful about before releasing this: the amount of
caching that the reverse proxy (nginx currently) will be doing when
forwarding the requests, and whether gzip compression has an effect on
the amount
progressive-loading-api-client: Proof-of-concept functioning api client with progressive loading.  Missing tests.
progressive-loading-api: Adds error handling, separator header & tests
Merge branch 'master' into progressive-loading-api
Merge branch 'progressive-loading-api' into progressive-loading-api-client

Conflicts:
	search/views/search.py
	static/js/languagelearning.main.js
	static/js/routers/languagelearning.js
	utils/api.py

Merging in updates from code review on `progressive-loading-api`, and bringing
the client code up to spec.  Also some cleanup besides.

Still missing client tests- those are next.
progressive-loading-api-client:

Added client tests for progressive loading.  Also updated sinon.js.
Merge pull request #28 from antoviaque/progressive-loading-api-client

progressive-loading-api-client: Proof-of-concept functioning api client ...
Merge pull request #27 from antoviaque/progressive-loading-api

progressive-loading-api: Progressively return API result while built