Display Curl commands in debugging messages

Legacy CLI clients of several services (python-novaclient,
python-swiftclient, python-cinderclient, ...) provide the user with a
--debug flag, displaying the equivalent curl command of each API call,
as well as the response received. This is very convenient, since it
allows developers to replay these commands for testing/debugging
purpose, without the need to craft the requests by hand.

On the other hand, with its --debug and -v flags
python-openstackclient doesn't provide such feature. While the --debug
flag shows tracebacks on error, the -v flag displays many debugging
messages, but the equivalent curl commands of API calls nor messages
exchanged.

This blueprint suggests that we implement a mecanism similar to the
one implemented in legacy CLI clients, to display equivalent curl
commands, along with messages exchanged with the services whenever
python-openstackclient calls an API.

Example:
$ nova --debug endpoints

REQ: curl -i 'http://11.22.33.44:5000/v2.0/tokens' -X POST -H
"Content-Type: application/json" -H "Accept: application/json" -H
"User-Agent: python-novaclient" -d '{"auth": {"tenantName": "demo",
"passwordCredentials": {"username": "admin", "password": "secret"}}}'

RESP: [200] {'date': 'Mon, 16 Dec 2013 10:09:04 GMT', 'content-type':
'application/json', 'content-length': '3939', 'vary': 'X-Auth-Token'}
RESP BODY: {...}

...