Account Store for OAuth 2.0 Access Token

[mrioan]

The Password Grant type implementation is ready to accept and AccountStore.

However we only have a DisabledAccountStoreResolver.

We must be sure we support this: https://docs.stormpath.com/rest/product-guide/latest/auth_n.html#generate-oauth-token.

It must also be documented in the Java Product Guide.

[dogeared]

related to multi-tenancy