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
Removes some swig workaround code put in for ticket 181.

This seems to fix the import problems recently seen on 12.10 for loading gnuradio-companion (due to 'from lxml import etree' which was actually due to 'import os' causing a segfault).
analog: adding squelch_base_ff and ctcss_squelch_ff to analog.

This addresses the issues of inheriting from both gr_block and another block (squelch_base_ff, in this case).
analog: adding complex squelch base class.
analog: implemented all AGC classes in gr-analog with QA code.
analog: added cpfsk to gr-analog.
analog: adding dpll_bb to gr-analog.
analog: adding GRC blocks for:

- AGC
- AGC2
- Feedforward AGC
- CPFSK
- CTCSS squelch
- DPLL
analog: added fmdet block to gr-analog.
analog: adding frequency_modulator_fc to gr-analog, with QA and GRC.
core: update to build_utils to help with '_i_impl' blocks.
analog: adding noise_source to gr-analog with GRC and QA.