#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.
Thinking about it, I'd guess we don't need the reflection here at all... Put in joda as provided dependency (not to be part of the shaded jar), and then we can simply instantiate everything. As you already checked the existence of the APIs in a safe way, this should be possible. After all, the classloading of the formatter class would only be attempted if you actually call ```validate(..)``` for the specific validator.