copy dagon into this tree

[johnynek]

https://github.com/stripe/dagon

Dagon was extracted from summingbird since we wanted to use it in scalding, but having a separate repo is inconvenient.

We should copy that code back here and move the package name to com.twitter.scalding.dagon.

This reduces external dependencies and challenges in maintaining.

[daniel-sudz]

I'm fine with doing this, seems pretty mechanical.

I would like to see some organization/splitting on the build.sbt file though just shoving everything in the root file is getting unwieldy. From just glancing at the docs I think a subproject build.sbt file should work