Added checks for missing comma after linking adverb on a new sentence

There are more sources to get help, but it seems only 1 url tag is allowed.
* http://www.grammarerrors.com/punctuation/commas-with-conjunctive-adverbs-however-furthermore-etc/
* http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv288.shtml

I also added the bigger list of http://grammar.yourdictionary.com/parts-of-speech/adverbs/list-of-conjunctive-adverbs.html but removed the ones which triggered positives with http://tatoeba.org/

Tested against a self-compiled 2.6.0-Snapshot with the standalone GUI.