Add ability to select username field from LDAP

[sroth80021]

This request originates from GitLab -- at present, the user names which I believe are provided by OmniAuth are coming into Gitlab using what appears to be the first cname= entry. (but I'm not sure how this is selected)

This means that in Gitlab, my name field ends up being of the form 'ROTH, STEVEN' (even though my email address is steve.roth@... and my Git name is 'Steve Roth') In our GitLab instance, we are seeing variances from user-to-user -- I see another user whose name is coming through in lowercase -- 'lastname, firstname', for example.

I think it would be very nice if OmniAuth/Gitlab supported specifying an arbitrary LDAP field to map to the 'Name' field. In our case, it seems the 'displayname' LDAP field would be the way to go, but I imagine that one might want to be able to customize this. Currently, I dont see any field in the OmniAuth LDAP readme by which this can be specified.

This would make the usernames more consistent and easier on the eyes.

[dan-blanchard]

This would be very nice. At present I'm having to tell all our users to go into their profiles and change the name field after the fact.

[pboling]

Documentation about how to use TLS certificate verification has been added to the readme:
#110