Adds choice of persistence level to Bagel.

This PR adds support for selecting the `StorageLevel` to use when running a Bagel computation. The default remains `MEMORY_ONLY` and should be fully backward-compatible in terms of `run` method calls.

Note that due to the way Scala handles default arguments it was not possible to simply add the `storageLevel` parameter to each `run` method with a default. Hence I did so for the "main" `run` method and then added a second method for each of the remaining ones, that takes care of the default argument (and whose signature matches the old versions).

See https://groups.google.com/forum/?fromgroups=#!topic/spark-developers/F_UOsKM8ZEM for the discussion.

I also added some documentation and a (probably superfluous) test to the `BagelSuite` to test use of the non-default `StorageLevel` parameter.