Add the Execution Monad
Get the tmp directory from the config
Add some tests
Add more zip methods and .unit to ease composition
Add Jco's writeThrough
Rename snapshotExecution to forceToDiskExecution for consistency
Merge with develop
Add Kmeans test to execution
Merge pull request #974 from twitter/execution_monad

Make an Execution[T] type, which is a monad, which makes composing Jobs easy.