Add Roundcube spk

Some decisions I made:

1) Name: roundcube vs roundcubemail. 
The project is called different names: "roundcube", "roundcubemail", "Roundcube webmail". Because this webmail project is the number one project on roundcube.net site, there are no other significant roundcube projects, roundcube is easier to read and event the name roundcubemail isn't used by them all the time I decided to use just roundcube.

2) SMTP server is set to default value "localhost" while IMAP is empty. 
It's because sending from localhost works just if it isn't blocked by ISP. IMAP host isn't set to localhost because by default no mailserver runs on Synology and users have the option to use any personal/work servers.