* Keep old entry points for preview plugins so people does not need to
do python setup.py develop
* Version checks should be done including the min_version parameter,
otherwise they will fail for 2.4, 2.5...
* No need to define different methods for different interfaces. If
running eg 2.3, the IResourcePreview interface methods will simply be
ignored.
* Simplified same domain checking
* Add note about these being eventually migrated to ckanext-geoviews
* Added docs
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