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
modtool: fixed order for both 3.6 and 3.7 API versions
Merge branch 'master' of git://github.com/gnuradio/gnuradio into grmodtool
core: adding variance calcs to perf. counters.

Using running mean/variance algorithm from Knuth's Art of Computer Programming.
core: adding new var perf. counters to swig interface.
core: Making an all C++ version of the preferences/config file readers.

This replaces the Python config parser stuff; should have same functionality that we need.
core: adding and using a PerfCounters section to the config files.

gr_block_executor will read the config files to determine if the perf counters are on or off.
core: only query preferences database once at start of block_executor.
core: add ability to override config files with environmental variables.

Env variables are named GR_CONF_<SECTION>_<OPTION> where the section is the [<section>] name from the config file and the option is the actual option name, all in upercase.
core: prefs of true/on/1 and false/off/0 are translated to bools.
docs: adding section to main page explaining config files.
core: fix gr_pdu_to_tagged_stream XML for type