Add authentication client
Add authentication client

Implemented a HTTP client toward the manager service. The client is
intended to be used as a library that will generate requests to the
'/identity' endpoint of a manager service instance selected through
provided URL.

Signed-off-by: Dejan Mijic <1e9694aae17a51d4875d9bf4a0d0e20ef37c1b85@mainflux.com>

drasko/mainflux,Mainflux/mainflux-lite,drasko/mainflux,drasko/mainflux