Simplify example execution and requirements

NOTE: omniauth 1.0.x doesn't return content_type
Fix parse_gecos to actually work
Move parse_gecos into private scope
Check also for empty values in gecos

value is nil if gecos string is shorter than gecos_map
value is empty if there is empty string in gecos (eg. 'name,,title,email')
Fix info.email to be present only if option email_domain is given

Option :email_domain could default to fqdn of localhost, though we
can't be sure that is correct.