Set default cookie path to app's base path instead of "/".

This allows running multiple apps in subfolders under document root, without
interfering with each other or potentially other non-cake apps on same domain.
Add "cookie_path" as shorthand for php.ini's "session.cookie_path" config.
Fix docblock
Update docblock.
Rename "cookie_path" to "cookiePath" for consistency.
Avoid unnecessary Hash::merge() call.
Update docblock.
Merge pull request #4275 from ADmad/3.0-cookie-path

Set default cookie path to app's base path instead of "/".