(2.7) Merge `datatype-jdk7` stuff in

[cowtowncoder]

Since jackson-datatype-jdk7 currently really only adds support for handling java.nio.file.Path, and since we have plans to update core dependency of databind to Java7 (JDK 2.7), it makes sense to just fold that handling in for 2.7.0, deprecate jackson-datatype-jdk7 after Jackson 2.6.