Fixes an issue with an empty/missing/null Email coming from GitHub's
OAuth call response.
Also, introduces the `sparse` index option on the User model's Email
field. This will ensure that we can have multiple User documents without
the Email field.
Adds a server-side User model test for the sparse index setting on the
email field.
Confirms that User documents without the email field are not indexed,
illustrating the sparse option on the schema's email field works
properly.
Added the dropdb task to the Gulp test:client & test:server tasks, to
ensure we have a clean database & that any indexes are rebuilt; this
will ensure any Schema changes (in this case the email index is rebuilt using
the sparse index option) are reflected when the database is started again.
Added a UPGRADE.md for tracking important upgrade information for our
user's to be aware of, when we introduce potentially breaking changes.
Included an explanation of the Sparse index being added, and how to apply it
to an existing MEANJS application's database.
Adds a script for dropping the `email` field's index from the User
collection.
Related #1145
