The jar with no name

This fixes #769.

This updates `JarStep` to only take keyword args, renames its `step_args` argument to simply `args`, and eliminates the `name` argument. The old behavior is all supported, but deprecated.

Also deprecated `MRJob.jar()`. My gloriously consistent plan is to eventually have `MRJob.steps()` only use classes from `mrjob.step`, which only take keyword arguments (`MRJob.mr()` will become `MRStep`; see #815).