Make hmac required argument in profiler.Profiler.init

In previous patch we changed behavior or profiler lib. Currently
profiler want create any notification if hmac key is not setted up
and all messages are not properly signed by it.

So to avoid mistakes (e.g. forgot to init profiler with hmac key) this
argument should be required (not optional)

As well a lot of improvments in docs related to profiler module

Change-Id: I74e05234a5638ffd93dbac9180d4497fcb5093ac