Document 'signBundle' bundler argument

This is a new argument for 8u40. It is positioned as a global argument, but it only affects the mac bundlers at the moment.

* This flag maps to a new -nosign CLI flag that sets the argument to false. No other flags were added and if the user wants to set a different value they can use the -BsignBundle=true option.
* There is a new signBundle attribute on the fx:deploy element in the Ant tasks.

The default behavior depends on the bundler. If it is set and a bundler does not support signing it may silently ignore it. If a bundler requires signing and it is set to false then a ConfigException will be thrown. Currently all bundlers that support signing default to true, although that may not always be the case as more bundlers support it.

Only Mac App, Mac Pkg, and Mac App Store bundlers support this value, and the default is true. For Mac App Store sigining is required. For all other bundlers (including Mac DMG and all Windows and Linux bundlers) the setting will be ignored regardless of the value.