add built-in logging function

[Karl-Philipp Richter]

which can be called in the PAC file and is printed to stdout. There should be no need for log levels and appenders because a PAC file is supposed to be rather trivial.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: libpacparser1 1.3.0-2
Uname: Linux 4.0.2-040002-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
Date: Thu May 14 19:59:01 2015
Dependencies:
 gcc-5-base 5.1~rc1-0ubuntu1
 libc6 2.21-0ubuntu4
 libgcc1 1:5.1~rc1-0ubuntu1
 multiarch-support 2.21-0ubuntu4
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-04-20 (24 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: pacparser
UpgradeStatus: Upgraded to vivid on 2015-04-24 (19 days ago)