Add a pluggable event publisher scheme

[vsbabu]

When each recording is done, raise one of the 2 events viz., ON_CHANGE, ON_FAILURE. ON_FAILURE is when we couldn't get the data. ON_CHANGE is when the recorded reading is different from previous reading.

We should be able to setup a JS registry file that calls a specific method for any source.source_key.ON_CHANGE stuff. The input to the method is the time, reading, delta from last reading and time of last reading.

[vsbabu]

Node has built-in events module that can be used for this. Just config enhancements and may be dynamic require are needed.