install typical distribution documentation

NEWS, PLATFORMS, README, AUTHORS, COPYING and COPYING.LGPL are now installed
under share/doc/libstatgrab
add support for system-wide logging properties

* when installing libstatgrab with log4cplus, enable search in sysconfdir
  for properties files, too
* deploy default properties when log4cplus enabled
Prepare for 0.91 release.

This includes a note in README that covers the RPATH/ldconfig issue
found #37, and also a note for the parallel installation discussion in
issue #25.