#224 checks for given dateFormat in DefaultConversionContext

DefaultConversionContext now verifies the dateFormat if given. Based on source/target type the formatter is derived and the dateFormat is check against that.
Direct instantiation did not work for all cases as the signature check for Java 6 did not pass for Java 8 Date/Time type. (Checking unresolved references to org.codehaus.mojo.signature:java16:1.0). So, I leave it as it is. merged and pushed to master