Supported include keys in gitblit.properties

This allows you to build a hierarchy of properties files or for a properties file
to include default settings.
Merged #211 "Fix raw servlet trashing paths with spaces"
Adopt chain-loading properties as the default setup
Merged #110 "Support multiple/chained config files"
Fix PluginManager not properly respecting --noverify
Documentation
Prepare 1.6.2 release
Merge release 1.6.2