Let "create server" set custom metadata

Fixes #37

When creating a server, custom metadata "os" and "users" are
automatically derived from the image properties. This commit
allows users to overwrite them or add extra custom metadata
at server creation time.
Escape control characters in HTTP body, when json

Fixes #32

This commit ensures that control characters are escaped in all
HTTP response bodies, as long as they are parsed as json strings
Raw content is kept intact
Response header contents were already handled correctly
Fix typo in server_create
Support more console types in get_server_console

Refs #36

New types: vnc-ws, vnc-wss
Support more console types in "server console" cmd

Refs #36

New types: vnc-ws, vnc-wss
Translate no-vnc console types from CLI to API

Fixes #36

The terms "no-vnc" and "no-vnc-encrypted" are expected to be easily
understood by users, while the equivalent terms vnc-ws and vnc-wss
(respectively) are used in Compute API. Kamaki CLI will now
accept both terms by internally translating the former to the later.