Enhenced Jackson#newObjectMapper to take optional JsonFactory

Enhenced Jackson#newObjectMapper to take an custom provided instance of com.fasterxml.jackson.core.JsonFactory. This allows further customizations of the ObjectMapper as allowing comments in JSON files...
Not having looked at the code for `ObjectMapper` in quite a while, does it throw a NPE if `jsonFactory` is indeed `null`? You don't have any type of assertion in the code to verify this claim.