Adding support for dropwizard 0.7.X configuration files

This is a first pass at adding dropwizard 0.7.X configuration file support which was the reason behind my original intention to upgrade `metrics-core` to 3.0.2. Let me know what you think.

It currently only supports `HttpTransport`. I'm trying to figure out a way to have add a transport key in the yaml configuration file where you can choose between http or udp.

For more on dropwizard configuration files for reporters, see [here](http://dropwizard.readthedocs.org/en/latest/manual/configuration.html#all-reporters).
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