Latest version support

[nehanarkhede]

Included a type VersionId that validates version specified using the string latest or an integer between [1,2^31-1]. Updated documentation to reflect that. This is a follow-up to and fixes #48

@granders Ready for a review.

@ewencp If you can review the docs to see if the versionId type makes sense, that'll be great. In particular, I'm not sure if we should stick with camel case convention for parameters or json field names for the REST APIs. I'm fine with either as long as it is consistent.

[ewencp]

@nehanarkhede for field names I'd usually go with underscores since that seems to be common convention for REST APIs, but I'm not sure there's a convention for field types since that's a language-specific issue. I agree that either is fine and I think it looks good as is.

[granders]

This looks good.

One question about the concept of latest - since this is inherently a mutable thing, we aren't really providing any consistency guarantees in terms of what a user will see when they query for "latest" right?

I guess in practice the KafkaStoreReader threads will keep all the servers up-to-date, so I don't know if this is worth mentioning in documentation or not.

[nehanarkhede]

@granders Rebased and modified the documentation to explain that. Will merge if you have no further comments.

[granders]

@nehanarkhede Looks good, thanks.