Protect against loading metadata from an old model version

[jasonnance]

Feature

Write the DLTK version number in the model metadata and either warn or raise an error if an incompatibility is detected when reloading an old model.

Motivation

Protects users from breaking changes in model metadata format and semantics.

Additional Details

Will need to determine what kind of breaking changes are possible to best know how to implement this. Maybe wait until we have to change something that would break backwards compatibility.