Automatically read config from jsduck.json file.

Check if jsduck.json exists in current working dir, if yes, then
read configuration from it.

Fixes: #335
Switch again back to using ExecJS middle-layer.

This is to allow third-party packaging of JSDuck without the
inconvenient dependency on therubyracer gem.

Fixes: #339
Add {DATE} and {JSDUCK} placeholders to --footer.

To easily insert current data and link to JSDuck homepage.

Fixes: #343
Add execjs dependency to .travis.yml.
Typo in command line option docs.
Up version to 4.7.0.
Make ThumbList work in IE10.

Temporarily patch the Ext.dom.Element#getAttribute method.
Up version to 4.7.1.
Merge branch 'master' into doc-tags
Improve auto-inheritance logic.

Remove the special-case rule of @deprecated tag.
Implement @localdoc tag.
Only auto-merge fields that aren't merged explicitly.

This solves the problem of @method getting an auto-detected :default
field which doesn't make sense for methods.