Introduced a mechanism for message signing and verification

* Every outgoing message can be optionally signed, and every incoming message can be optionally verified
* Messages in Timers example now use this mechanism to demonstrate the way it works
Added class allowing to measure and calculate some timing statistics.

* Stats class that makes creating timing statistics (based on elapsed time) easier
* Refactoring of Timer class (you can now get the current timestamp in both milliseconds and seconds)
Logger now supports different log levels.

* Based on the value of LOG_LEVEL, different logger functions are turned off or on