Added ability to specify global tags as part of configuration

In our environment we only want to run a single instance of the DogStatsD agent.  We then need to configure each server to tag their stats as being related to a particular host so that we can filter and graph across hosts.  This pull request allows you to set a tag at config time that will be applied to all stats.  We are adding a tag called hosts with a value of Naming.CurrentHostName so that all stats are tagged with this.