Add GPL v3 license to the project

• [DH] LICENSE

Add validation to vpn service (refs #20)

• [DH] services/vpn/handler.py
• [DH] services/vpn/host.py

Add validation to DnsHandler (refs #20)

• [DH] services/dns/host.py
• [DH] services/dns/mx.py
• [DH] services/dns/ns.py
• [DH] services/dns/zone.py

Add validation to DhcpHandler (refs #20)

• [DH] services/dhcp/host.py

Validate index on XxxSubHandler methods (#refs 20)

• [DH] pymin/service/util.py

Add common Int related validators (refs #20)

Is very common to validate a number that must be in the range of an
(unsigned) integer of 8/16/32/64 bits. Int8, UInt8, Int16, UInt16, Int32,
UInt32, Int64 and UInt64  are provided for that.

• [DH] pymin/validation.py

Add IPAddress validator to pymin.validation (refs #20)

• [DH] pymin/validation.py

Add host name related validators to pymin.validation (refs #20)

This add classes HostName and FullyQualifiedHostName.

• [DH] pymin/validation.py

Improve and document UpOneOf validator (refs #20)

• [DH] pymin/validation.py

Move common validation code to new pymin.validation module (refs #20)

This module includes all the validators from formencode plus some custom
validators specific to pymin.

• [DH] pymin/validation.py
• [DH] services/firewall/rule.py

Use ip command to configure VPN networks in vpn service (closes #34)

• [DH] services/vpn/handler.py
• [DH] services/vpn/templates/tinc-up

Use more meaningful names in Vpn items from vpn service (closes #35)

• [DH] services/vpn/handler.py
• [DH] services/vpn/templates/tinc-up
• [DH] services/vpn/templates/tinc.conf

Import HandlerError in vpn service handler (closes #32)

• [DH] services/vpn/handler.py

Rearrange Host attributes from vpn service (closes #33)

The attribute names are named after the tinc configuration variables to
avoid confusion and the name of the host is now the HostHandler key.

• [DH] services/vpn/host.py
• [DH] services/vpn/templates/host

Add an example of regular attributes usage with a ValidatedClass

ValidatedClass were only used with Field instances as attributes.
Sometimes regular (not validated) attributes are needed.

• [DH] pymin/validatedclass.py

Call ComposedSubHandler._attr() with cont parameter (closes #31)

• [DH] pymin/service/util.py

Convert serializer data to unicode in PyminDaemon (closes #28).

• [DH] pymin/pymindaemon.py

Remove invalid TODO comment in proxy host subhandler (closes #8).

DictSubHandler fits just fine for proxy host management.

• [DH] services/proxy/host.py

Split proxy handler in submodules (refs #2).

• [DH] services/proxy/handler.py
• [DH] services/proxy/host.py
• [DH] services/proxy/user.py

Split vpn handler in submodules (refs #2).

• [DH] services/vpn/handler.py
• [DH] services/vpn/host.py