4.7.0 Make JSDuck runnable outside of Rubygems ecosystem

• Remove needless require 'rubygems' statements from source.
• Use ExecJS to allow other JavaScript runtimes besides therubyracer (e.g. nodejs).
• Automatically read config from jsduck.json file, if present in current dir.
• Add "Open in new window" button to basic (non-touch) examples page.
• Add {DATE} and {JSDUCK} placeholders to --footer.
• Add --message option for displaying prominent warning message in docs.
• Add --debug option (equivalent of --template=template --template-links).