Added `share_dir` global config setting and now using it as default
for read only resources needed by various plugins.
Update docs to reflect use of share_dir-based defaults for certain plugin
config values.
Merge branch 'config-doc-reorg' into share_dir

Conflicts:
	CHANGES.txt
static_directory setting from "dashboard" -> "dasher". Can't change
the plugin settings w/o also changing the build.
Merge pull request #679 from mozilla-services/share_dir

Share dir