License is now Apache + the original DataDog license,
i.e., dual licensing
Add the DataDog license and copyright to all files that have
the Apache license
Added that code was forked from Datadog's dogstatsd-python to
README.md
Cherry-picked from master although this commit adds licenses to
files that had already been deleted in master
Change-Id: I4117ad87fbe6c3ffa067b66463d636912dc39d3a