core: Adds a gr-log concept for easy logging at various levels.

Uses log4cxx and is optional. Macro based and macros are simple pass-throughs
if this dependency does not exist or if the gr-log is disabled.
docs: adding info on gr-log feature.
digital: Adds checks to enable use of gr-log in digital blocks.
Fixing up the gr-log capabilities.

Better handling of enable/disable function, easier integration with all components if log4cxx is or is not there. All components have been updated to be able to use logging. Docs update, too.
howto: adding gr_log checks to build.
log: Restructure config file handling for gr-log.

The xml config file is installed into prefix/etc/gnuradio.

A new LOG section is added to gnuradio-core.conf that has an item log_file to point to the log file to be used.

If the log_file is not found, we can now set the file to "" to use the BasicConfigurator for very simple console logging. This is what will be used during 'make test'.

In gr_fir_sysconfig_x86.cc, all std::cerr calls are replaced with gr-log to easily turn them on/off.
log: added a macro and routine to set log level from config file.

Also worked on documentation of logging features.
log: fixed gr_log to be usable in Python.

Works with log4cxx installed and not installed.
Merge branch 'master' into gr_log

Conflicts:
	CMakeLists.txt
	cmake/Modules/GrMiscUtils.cmake
	docs/doxygen/other/build_guide.dox
	gnuradio-core/CMakeLists.txt
	gr-digital/lib/CMakeLists.txt
blocks: moving unpack_k_bits to gr-blocks.
blocks: moved pack_k_bits to gr-blocks.
blocks: added gr::blocks::tuntap_pdu

Linux only
Merge branch 'wip/pdu-tuntap'

Conflicts:
	gr-blocks/include/blocks/CMakeLists.txt
	gr-blocks/lib/CMakeLists.txt
	gr-blocks/swig/blocks_swig.i
blocks: added gr::blocks::socket_pdu
blocks: added gr::blocks::pdu namespace for PDU functions
blocks: added gr::blocks::pdu_to_tagged_stream
blocks: added gr::blocks::tagged_stream_to_pdu
blocks: moving argmax and max to gr-blocks.
log: fixups after merge.
log: cleaned up naming, added gr_log_get_level function.

Changed the name of the logger in the xml to use a better name.