Add support for graphite tags

[frfoe]

To use tags at least the following options must be set in carbon.conf:

• ENABLE_TAGS = True
• GRAPHITE_URL to the URL of graphite-web (e.g. http://127.0.0.1)

A boolean parameter gr_tags_enable should be added to class graphite with a default vale of false. If set to true and is true and gr_carbon_ver > 1.1.0, the above options are set in carbon.conf. GRAPHITE_URL will be set to http://127.0.0.1 + the value of gr_web_server_port.