Add Offline Support to Structure101 Plugin

[jzheaux]

When trying to build Spring Security when disconnected from the Internet, the Structure101 plugin complains with:

Unknown host 'structure101.com: Temporary failure in name resolution

The plugin at least should have a way to be turned off when in offline mode.

Or, perhaps better, it may be possible to run without a connection if, for example, Structure101 is already installed locally. I'm not sure about this as the Structure101 library may require a connection to verify the license.