Introduce basic support for IPv4 classful networks

[rheone]

Description

Arcus is missing in-the-box support for Subnets defined by and expressing traits of the accepted IPv4 Classful Network. It is desired that Arcus should provide legacy support for A, B, C, with a possibility for D (multicasting) and E classful networks.

Arcus should:

• Allow the Subnet object to be constructed, either via constructor or a factory, by issuing an appropriate IP Address and class specifier
• Subnet should have a property dictating its class if it is IP4 and meets the appropriate class
• It should be possible to translate between IP Address netmasks and class A, B, and C