Applied word wrap to pop3_connector module documentation. Refs #2954
POP3 connector manual updated. Refs #2924

Added index, references, gui labels and interface description.
POP3 Connector: added References section. Refs #2924
"download site" macro as labeled link.
Whitespace and formatting changes
Moved "install from USB stick" to Install from CentOS section.
Added section names and index entries
Fixed Contacts section
NethServer Enterprise: rebased installation section

Use rst_prolog macros for differences with the base version.
Warning for Active Directory users. Refs #2954
Merge pull request #44 from DavidePrincipi/master

POP3 connector update
Merge pull request #45 from DavidePrincipi/install_usb

Install USB
Bump version 6.6.

Removed social-media contacts from the front-page: only a link to the
official web site.
New Server Certificate and Organization contact sections. Refs #2969