Datadog histograms and tags support

Hi.

I added support for datadog [histograms](http://docs.datadoghq.com/guides/metrics/#histograms) and [tags](http://docs.datadoghq.com/guides/metrics/#tags) features.

I partly used [code from Young Han Lee's fork](https://github.com/joybro/node-dogstatsd), but I preserved backward compatibility with existing API.

I added optional `tags` parameter to all functions.

I also allowed users to skip `sampleRate` parameter, passing `tags` or `callback` instead of it.

The only function which is not 100% backward-compatible is `send`, because I thought it internal.

I also marked all optional parameters in docstrings with `=` symbol according to [JSDocs giude](http://usejsdoc.org/tags-type.html).

I added tests for new functionality, but I haven't documented it in `Readme.md` yet.