[WIP] Neutron v2

This pull request introduces OpenStack Neutron v2 support. Attempts have been made to keep things in line with a few of the new idioms (pagination, service clients, etc.) but I very much want to hear people's feedback if certain things look wrong :smile: 

**API versions**
- [ ] List Neutron API versions
- [ ] Show details for a particular API version

**Extensions**
- [ ] List Neutron extensions
- [ ] Show details for a particular extension

**Networks**
- [ ] List networks (including marker pagination and filtering by query parameters)
- [ ] Create a network
- [ ] Update a network
- [ ] Show details for a particular network
- [ ] Delete a network

**Subnets**
- [ ] List subnets
- [ ] Create a subnet
- [ ] Update a subnet
- [ ] Show details for a particular subnet
- [ ] Delete a subnet

**Ports**
- [ ] List ports
- [ ] Create a port
- [ ] Update a port
- [ ] Show details for a particular port
- [ ] Delete a port