Merge pull request #24 from martinleopold/master

support for attr- fields in iframe cell
Merge pull request #31 from martinleopold/master

Updates to issues #15, #25, #30
Merge pull request #34 from martinleopold/master

Some error handling (no JS, API availability)
Merge pull request #37 from martinleopold/master

Added error pages
Merge pull request #39 from martinleopold/master

quick fix for #38
show / hide poster frame in iframe cells on activate / deactivate