Add isInRole to JS API as well

Something you frequently do in the application, is check if the current user is allowed to access something. As there currently is no shorthand for this in the Javascript API, you have to do something like:

if ( Roles.userIsInRole( Meteor.userId(), 'permission' ) )
    // ...

I noticed the UI helper offers a beautiful shortand syntax for this, using the currently logged in user. I think it would make a lot of sense to add this to the API as well, so that one can simply do:

if ( Roles.isInRole( 'permission' ) )
    // ...

or even better:

if ( Roles.has( 'permission' ) )
    // ...