Changes to packages/callback-hook:
- Add bindEnvironment option to Hook constructor (defaults to true).
- Add internal helper function dontBindEnvironment() which does all the
error handling stuff like Meteor.bindEnvironment() but doesn't change
the environment.
- Use dontBindEnvironment() instead of Meteor.bindEnvironment() when
bindEnvironment option is false.
- Add tests.
Changes to packages/accounts-base:
- Create hooks with { bindEnvironment: false }
- Add test for whether Meteor.userId() is available in login callbacks.
