Add support for resources with dashes in name

[julozi]

Proxmox API includes a few resources with dashes (-) in their name (like /api2/json/nodes/{node}/qemu/{vmid}/agent/get-host-name).
However the dash character (-) is not authorized in Python object attributes.

This PR adds support for this kind of resource's name by replacing dashes by underscores.

Exemple code :

client.nodes('my_node').qemu(12).agent.get_host_name.get()

will call the API URL : /api2/json/nodes/my_node/qemu/12/agent/get-host-name

[coveralls]

Coverage increased (+0.07%) to 79.208% when pulling de51882 on julozi:support_dash_resources into f7874b5 on swayf:develop.

[morph027]

@julozi Would you mind re-creating this PR for https://github.com/proxmoxer/proxmoxer ?