extracted logic from ConfluencePluginList constructor

...as the default constructor gets called implicitly by sub-classes,
which leads to errors if no internet connection is avaiable.
Merge pull request #13 from sjka/offline
Fixed offline capability
minimize differences from the upstream.
Merge pull request #15 from ikedam/feature/13_offline_with_nowiki

 Minimize differences from the upstream of #13 .