DATACMNS-447 - Soften too strict check in MappingContext if no PersistentEntity type.

Previously we directly threw a MappingException in MappingContext.getPersistentEntity(TypeInformation) when the MappingContext was in strict mode and we were given a TypeInformation for which we didn't have a PersistentEntity already.
We now first check whether we should actually create a PersistentEntity for the given TypeInformation if no PersistentEntity is available, before throwing a  MappingException - if we should not then we simply return null (e.g. for simple types like Integer or Double).
Opened-up API for customization in sub-classes.

Original pull request #66.