Added POP3 connector link to "Enterprise".
Updated Enterprise migration.rst
Fixed Enterprise title
Fixed mail References section
"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.
Update gateway.rst
Developer manual: add collectd chapter
Administrator and developer manual: add statistics (collectd) chapter
Improved the explanation of LDAP configuration

Ldap configuration is useful only after updated from version 5
Admin manual: update migration chapter
Admin manual: a^C warning for MSSQL support not implemented yet
Merge pull request #44 from DavidePrincipi/master

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

Install USB