Enables Exercise List posting

Turns possible sending a list of Exercise prototypes as a POST. Although, it lacks the returning ID as response yet
Response to Posting a List now has IDs
Fixes #11 at all.

The code urgents to get better but that will depend on some research on the code of endpoints-proto-dastore. It is a lot hardcoded now, but it works.
Solves the problem with the best way
Merge pull request #30 from AEHO/feat/post-a-list

Fixes #11 (POST with list).