Use 'official' elasticsearch client instead of rawes

https://github.com/NLeSC/xtas/issues/13: I've rewritten the calls to rawes in tasks.py to use the elasticsearch-py client maintained by elasticsearch. I think this is better because it is more 'official' and very close to the raw API, and it also supports _update so the store task is now much easier. 

I've also added unit test cases for the backend calls.

(I guess it would have been better to create a separate branch for the pull request on my side?)