Upgrading client-sessions to latest release.

The new release contains the pull requests I made, so we're good to go!
Adding new expansion options.

These let developers choose whether or not they want to auto-expand linked
account attributes. This makes coding more convenient, but comes with a slight
performance penalty as these values must be fetched from the cache on each
request.
Defining some jsdocs for the accountCallback stuff.
Adding an expandAccount helper.

This allows us to prefetch expanded fields on behalf of the developer.
Modifying getUser so it properly expands account details.
Updating changelog.
Adding upgrading docs.
Updating stormpath.
Adding automatic expansion docs to the product guide.
Adding blocking for .gitignore.
Bump.
Bump.
Using normal errors.
Including expansions when first fetching accounts.