Add support for stdlib LogFormatter in LogService

[tcalmant]

Currently, the log service doesn't support formatters: it is necessary to create a LogListener to print out logs with a custom format..

Solutions can include:

• add support for a formatter in the log shell commands (basic print out)
• mimic the standard library LogEntry members for a Formatter to handle entries from the Log Service
• add a basic log listener component which could mimic a Handler (with its own Formatter) to ease log writing.

Requested at: https://groups.google.com/d/msg/ipopo-users/7tmyroWUEHg/a4w3mkwEAwAJ