Mavenize Sphinx docs

This PR mavenizes the Sphinx documentation of Dropwizard and adds the Maven GitHub Site plugin to generate and publish Dropwizard documentation (as well as the Maven sites) automatically.

I've added a POM file to the `docs/` subdirectory containing the Sphinx documentation and included the [sphinx-maven plugin](http://tomdz.github.io/sphinx-maven/) to build the actual documentation. I've also added the [GitHub Site plugin](http://github.github.com/maven-plugins/site-plugin/) to automate publishing the generated documentation and Maven sites.

The result of a test run can be found at:
* Contents of the `gh-pages` branch: https://github.com/joschi/dropwizard/tree/gh-pages
* Rendered docs: http://joschi.github.io/dropwizard/0.7.0-SNAPSHOT/docs/
* Maven site for `dropwizard-core`: http://joschi.github.io/dropwizard/0.7.0-SNAPSHOT/dropwizard-core/

Current short-comings:
* Upload of the Maven sites and the documentation is implicitly run during the `site` phase. To disable the upload you have to add `-Dgithub.site.dryRun=true` to your Maven run.
* Hyperlinks of the Maven sites are incorrect. This might be fixed with the [relativizeDecorationLinks](https://maven.apache.org/plugins/maven-site-plugin/run-mojo.html#relativizeDecorationLinks) parameter of the maven-site-plugin.
* Project reports like the generated Javadoc, FindBugs, or PMD results are missing. Those can and should be activated in the parent POM.
* The ugly standard skin is being used for the Maven sites. Instead we should use the [Fluido skin](https://maven.apache.org/skins/maven-fluido-skin/). :wink: