add choice 'service_and_name' for 'tag_for' option

This pull-req enhances `tag_for` option to add 'service_and_name'.
It is very useful for these points.

* it could use service name from tag and graph name together.
* on embedding image at external page, it is user friendly for including service name in the image.

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### sample for `tag_for service_and_name`

![gf-tag_for-service_and_name](https://f.cloud.github.com/assets/1734549/1066201/bad137f0-13ad-11e3-8650-3cdea34fcce4.png)

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### sample for `tag_for service`

![gf-tag_for-service](https://f.cloud.github.com/assets/1734549/1066202/baf38b2a-13ad-11e3-9027-5b84a6a80af3.png)