add -graphvizdot support

If the user installs graphviz with macports in Mac, the `dot` will be installed in `/opt/local/bin/dot`. But plantuml will invoke `dot` in `/usr/bin/dot`. So there will be some java errors during octopress generation.

My solution is to add a configuration parameter in _config.yml called 

```
plantuml_dotpath
```  

to specify the `dot`  command path.
And then add the `-graphvizdot` option with the plantuml.jar.

What do you think?

BTW, maybe we need to notify the user to add the `public/images/plantuml` path in the clean task in Rakefile. Otherwise, the error picture will not be removed after generated.