Adjust accounts-twitter to use Twitter's id_str instead of id

This is a PR to resolve issue #629.

Suggestions for release notes (3rd one explains a potential breaking change):

- When someone signs in with Twitter, their Twitter User ID will now be stored as a string.  Previous versions of Meteor stored it as an integer. This change is in preparation for Twitter's User IDs getting longer:  https://dev.twitter.com/blog/64-bit-twitter-user-idpocalypse
- Existing users with integer ids will be found as the user signs in, and their ID will be updated to a string.
- Meteor Apps that look up users by Twitter User ID should update their code to look them up by the string representation of the ID.  Twitter provides this in their user objects as id_str.