Adds support for domain user logins

The user name can now be specified as UPN (e.g. user@domain.com)
or down-level logon name (e.g. DOMAIN\user).

URL decoding has been added for form elements as well,
although this should be handled in EHS.
Merge branch 'master' of https://github.com/cloudbase/FreeRDP-WebConnect
Merge pull request #61 from cloudbase/master

Adds support for domain user logins