Implemented logging.

Added class PsrLoggerBridge, which connects PSR3's LoggerInterfaces to
specified RestHttp's logging events.

Implemented FileLogger.
Fixed error when logging is enabled.

Fixed error in processing of Http request when logging is enabled and
request body is empty.
Merge remote-tracking branch 'upstream/master' into logging
Merge pull request #4 from rpetrano/logging

Implemented logging.
Merge branch 'master' of github.com:ngs-doo/dsl-client-php
Fixed logger typo

log=>Log in Requirements.php
Multiple find, history

Multiple find and history are PUT request