synchronise access to plugin instance variable searchProviders to solve
a multithreading issue.
update parent pom to 2.7
update celements-core to version 1.91
add missing context to openSearchers
allow connected threads to reconnect to a markToClose SearcherProvider
change getSearcherProvider to getConnectedSearcherProvider and ensure
that the searcherProvider is connected before returning
add java doc
Merge pull request #2 from celements/fixSearchProviders

Multithreading bug with accessing this.searchProviders