Implement network calls in nova client

Provide network API calls in nova client as well as unit tests. API is accessed by `networks` field of nova client.
Methods:
* list() - return network list
* get(network object or id) - return requested network
* delete(network object or id) - delete requested network
* create(network properties) - create a new network with requested properties
* disassociate(network object or id) - disassociate the network from its project
* add(network object or id[optional]) - associate the current project with a network