added /session/status endpoint

[dannycoates]

This adds an endpoint to check whether a sessionToken is still valid.

r?

fixes #610

[ckarlof]

r+ from a design perspective.

[rfk]

Tiny nit: I found "while the request returns..." hard to parse as it suggests some sort of long-poll or iteration; suggest rewording along the lines of "The request will return a success response as long as the token is valid"

[rfk]

r+

Although I'm not thrilled about adding new things to the /v1 API without revving a version somewhere. I guess we can still get away with it at this point, but I'll feel a little dirty inside.

[chilts]

Tests pass locally. Code looks fine. r+ with @rfk's comment about the doc.

On the mention of the API version number, we need to come up with some guidelines perhaps on when to bump. Since this is adding new functionality, it's backwards compatible (AWS do a similar thing, don't bump unless it changes something).

I'm not fussed either way, but clarifications might help now and for the future. :)

[rfk]

Since this is adding new functionality, it's backwards compatible

This is fine while we're running the single canonical instance of the API, but it gets a lot muddier once people start running their own servers. Let's take it to the mailing list.