first cut at global rc files
forgot rc._update(rcdict)
Merge branch 'scopatz/glbrc' into master

* scopatz/glbrc:
  forgot rc._update(rcdict)
  first cut at global rc files
Merge branch 'master' into clang

* master:
  forgot rc._update(rcdict)
  first cut at global rc files
Factor globalrc parsing into utils.parse_global_rc

This allows us to parse global .xdressrc files from unit tests,
which is required for broken clang installs.
Pull clang_includes out of global .xdressrc

This avoids the need to hack clang_parse for broken clang setups.  The
unfortunate developer can now set 'clang_includes' in .xdressrc instead.