Add configurable outputWriters

I've added the ability to specify outputWriters in the YAML:

outputWriters:
    - "@class": "com.googlecode.jmxtrans.model.output.GraphiteWriter"
      settings:
          host: "localhost"
          port: 2003

NOTES:
- I've attempted to be completely backwards compatible.
- PLEASE test this before running in production
Merge pull request #67 from tewner/yaml2jmxtransConfigurableOutputWriters-Take5

Add configurable outputWriters