add option graph_path for more flexible URL designing

This pull request adds option 'graph_path' to customize flexible URL designing.
It could possible to switch 'graph_path' from 'tag_for' completely.

----------

### sample1

```
<match test.Foo>
  type growthforecast
  gfapi_url http://growthforecast.local/api/
  graph_path ${tag}/response_time/${tag}_${name}
  name_keys 0-100msec_percentage
  remove_prefix test
</match>
```

![gf-tag_for-service_and_name](https://f.cloud.github.com/assets/1734549/1066201/bad137f0-13ad-11e3-8650-3cdea34fcce4.png)

----------

### sample2

```
<match test.Foo>
  type growthforecast
  gfapi_url http://growthforecast.local/api/
  graph_path ${tag}/response_time/${name}
  name_keys 0-100msec_percentage
  remove_prefix test
</match>
```

![gf-tag_for-service](https://f.cloud.github.com/assets/1734549/1066202/baf38b2a-13ad-11e3-9027-5b84a6a80af3.png)