0.5.0

This is the first "release", and is the culmination of a lot of work.
Some highlights of recent developments leading up to this version:

• rename metric-tank -> metrictank
• promote metrictank to top level of repo and move other stuff out, start versioning metric schema's in other repo
• add kafka-mdm, kafka-mdam and carbon input next to nsq.
• accept empty inputs instead of calling log.Fatal #226
• don't busy loop and max out cpu if metricpersist production has issues
• make sure metricpersist msgs can't go over nsq's max size. #224