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.
channels: adding GRC tone example
channels: fix for phase imbalance/alias in the specular component for Rician channels, switch phase accumulator to double precision, exporting step size to grcp
qtgui: adding new features.

1. Using checkable menu items to toggle things like autoscale on/off.
2. Adds ability to put time plots in semilog-x and semilog-y mode.
qtgui: tweaking plotting apps for better handling.
qtgui: improved handling of GUI events and log-y scale.
ctrlport: new DISPLOGTIMESERIES{F,C} to set plot into log mode.
ctrlport: adding display type for stripchart as opposed to timeseries.

Stripcharts add data to the back of what's plotted; time series plots just plot the vector given. All single-value RPC vars have been converted to use stripchart mode.
Merge branch 'master' of gnuradio.org:gnuradio
Merge branch 'cpp_prefs'
core: if the config files don't exist, just return an empty list; don't fail.
Merge branch 'next' of gnuradio.org:gnuradio into next

Conflicts:
	gr-channels/lib/fading_model_impl.cc
Merge branch 'master' into next

Conflicts:
	gnuradio-core/src/python/gnuradio/gr/__init__.py
ctrlport: fixing typo in gr_throttle RPC export.