Move the datadog http client to a separate module

[amalakar]

I believe the datadog http client is valuable outside of the context of being an adapter for codahale metrics. There aren't any other http client for datadog in java/scala. I would love to use it outside the context of metrics-datadog. Thoughts?

[nickdella]

Yeah, I was surprised to see that there's no 'official' Java client for the Datadog APIs, so this would be a move in that direction: http://docs.datadoghq.com/libraries/ . We just went to a multi-module build thanks to #17 (thanks @tianx2!), so extracting the http client shouldn't be too difficult. I welcome your PR :)

[nhoughto]

+1 for this, we're excluding http4 and avoiding classes that import it to avoid the classpath bloat.

Also i don't think the io.dropwizard:dropwizard-metrics module is required either? i can't see where its used? That dependency brings in guava too which is whole other set of bloat.