Add merge_users helper
Add merge_users helper

```
$ sentry merge_users dcramer@gmail.com david@getsentry.com
Found 1 unique account(s) with duplicate identities.
Merge david@getsentry.com into dcramer@gmail.com? [Yn] y
david@getsentry.com was merged into dcramer@gmail.com
```

mvaled/sentry,zenefits/sentry,mvaled/sentry,fotinakis/sentry,daevaorn/sentry,zenefits/sentry,mvaled/sentry,zenefits/sentry,daevaorn/sentry,beeftornado/sentry,zenefits/sentry,JamesMura/sentry,jean/sentry,BuildingLink/sentry,fotinakis/sentry,mvaled/sentry,nicholasserra/sentry,alexm92/sentry,gencer/sentry,mvaled/sentry,fotinakis/sentry,JackDanger/sentry,daevaorn/sentry,mitsuhiko/sentry,jean/sentry,gencer/sentry,daevaorn/sentry,BuildingLink/sentry,jean/sentry,mitsuhiko/sentry,JamesMura/sentry,looker/sentry,JamesMura/sentry,gencer/sentry,beeftornado/sentry,ifduyue/sentry,nicholasserra/sentry,alexm92/sentry,gencer/sentry,jean/sentry,ifduyue/sentry,JamesMura/sentry,JamesMura/sentry,fotinakis/sentry,ifduyue/sentry,alexm92/sentry,jean/sentry,looker/sentry,looker/sentry,nicholasserra/sentry,JackDanger/sentry,looker/sentry,BuildingLink/sentry,BuildingLink/sentry,ifduyue/sentry,zenefits/sentry,JackDanger/sentry,BuildingLink/sentry,mvaled/sentry,ifduyue/sentry,gencer/sentry,beeftornado/sentry,looker/sentry