Store the version of the namespaces in the cached specification

[t-b]

Brought here from https://github.com/NeurodataWithoutBorders/pynwb/pull/953#discussion_r286616836.

[oruebel]

@t-b to clarify, the version of the namespace is stored in the cached specification as part of both the JSON of the namespace and the path where the namespace is stored. What is missing is that we currently do not store a version key for the individual types. This is in so far not a problem in that we assume that a file is written with one version of a namespace, rather than mixing versions within the same file. However to be explicit and prepared for the future it should be stored.