Added http_utils, HTTP client utilities.

How to use it:
    from osclient2 import http_utils as http
    metadata, body = http.get('http://openstack.jp/')

It has functions to access HTTP server normally:
* get(url, headers={})
* head(url, headers={})
* delete(url, headers={})
* post(url, body=None, headers={})
* put(url, body=None, headers={})

And it has useful functions to request/receive JSON body:
* get_json(url, headers={})
* post_json(url, body='', headers={})
* put_json(url, body='', headers={})
Added basic context class structure and Keystone v2 driver.

How to use it:
    from osclient2.context import Context
    c = Context(auth_type='keystone_v2',
                auth_url='http://192.168.0.11:5000/v2.0/',
                user='username', tenant='projectname',
                password='password', region='regionname')

auth_type parameter is the dispatcher of Context class. Now
keystone_v2 is only supported and the default value.

Other parameters are driver specific. If you use keystone_v2 driver,
you can omit all parameters with credential environment variables
like OS_AUTH_URL, OS_USERNAME and so on.