Add token based authentication to Activiti

At the moment the only way to sign into the Explorer is via username/password authentication,
we suggest adding a token based authentication to that.
The basic idea is that a you can generate login tokens using the IdentityService, these can
be used e.g., to generate notification E-Mails with a direct login url, or that another
application can request such a token via the REST interface so that the user does not
have to authenticate again.

Of course certain security measures need to be implemented e.g., limited lifetime/uses of a token.

We are willing to provide the implementation if it will be included into Activiti.

Any comments?