upgraded grails wrapper to 2.2.4
added a definition of Maven central repository
ignored IntelliJ IDEA project file
extracted a private method into Util and added test cases
tweaked description
tweaked introduction.gdoc
tweaked optimisticLocking.gdoc
tweaked withPessimisticLock.gdoc and withOptimisticLocking.gdc
added about "Field Error Binding"

https://github.com/kobo/grails-domain-locking/issues/3
version 0.4
fixed a bug that checking if there's no configuration in Confing.groovy is wrong
tweaked OptimisticLockingSpec