Update identity from Github.com

Since a huge majority of `github` identities are from Github.com, having a possibility of automatically updating a `github` identity (and its profile) with its current status in Github.com would be very useful.

The function could be used for batch update scripts as well as from the web UI.

Given a `github` userid, the function would query its profile at Github.com and...

- If the combination of values in the key fields of identities (email, name, userid) does not exist, a new identity should be created under the same profile.
- Any field with a non-empty value in Github.com and an empty one on SortingHat, should be filled in SortingHat
- The LinkedIn identity is unique, so
- If it already exists in SortingHat, merge the profiles.
- If it doesn't match the value in SortingHat's profile, split the identity to a new profile with the new LinkedIn link.
- Enroll to the company field, if not empty in Github, or already existing in SortingHat.