Use logfmt

[mutablestate]

This PR converts the current Logger entries into logfmt (reference to issue #84)

• add a LogEntry module to define a struct with defaults
• replace the Logger module with LoggerUtils (uses transformational functions rather than wrapping the Elixir Logger calls)
• convert current Logger calls to use the new Util functions
• add doctest and update current logger tests
• update docs and moduledocs

[riverrun]

That's great. Thanks!