Add `group` call

Added the group method for the Vero integration.

Signed-off-by: Nathan Houle <nathan@nathanhoule.com>

Remove trailing comma

Change API token to testing account

Add test fixture for #group call

Enable strict mode

formatting fix

Convert group tests to use fixtures

Test mapping email

Test falling back on email address

Add missing reject dependency

Change mapper
Merge pull request #1 from segmentio/feature/group

Add `group` call