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
core: fix gr_message_debug for printing PDUs
Merge branch 'maint'
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.
core: added missing include for gruel thread
blocks: added missing include for gruel thread
Merge branch 'maint'