howto: copies Doxyxml from main docs to properly pull out block parameters documentation.
GRC: adds ability for grc to pull in documentation for blocks from other GR modules.
Merge branch 'maint'
docs: adding an exclude rule for gr-modtool to avoid conflicts.
docs: correcting gr-modtool exclude path for master dir format.
core: prefs setup looks for a local .gnuradio/config.conf to use.

Order of precedence:
env. vars -> .gnuradio/config.conf -> ${prefix}/etc/gnuradio/conf.d/*
core: removed vestigial debug line.
digital: moved simple_correlator (inverse of simple_framer) to gr-digital.
blocks: adding count_bits code to gr-blocks.
digital: adding simple_correlator to gr-digital's block tree.