Add support for JSON readers

[dborovcanin]

FEATURE REQUEST

1. Is there an open issue addressing this request? If it does, please add a "+1" reaction to the
   existing issue, otherwise proceed to step 2.
   No.

2. Describe the feature you are requesting, as well as the possible use case(s) for it.
   Mainflux writers will support storing arbitrary JSON using JSON transformer. Both Mainflux Readers interface and implementation need to be modified to add support for reading JSON messages.

3. Indicate the importance of this feature to you (must-have, should-have, nice-to-have).
   This is a must-have.