spark-729:  predictable closure capture

SPARK-729 concerns when free variables in closure arguments to transformations are captured. Currently, it is possible for closures to get the environment in which they are serialized (not the environment in which they are created).  This PR causes free variables in closure arguments to RDD transformations to be captured at closure creation time by modifying `ClosureCleaner` to serialize and deserialize its argument.

This PR is based on #189 (which is closed) but has fixes to work with some changes in 1.0.  In particular, it ensures that the cloned `Broadcast` objects produced by closure capture are registered with `ContextCleaner` so that broadcast variables won't become invalid simply because variable capture (implemented this way) causes strong references to the original broadcast variables to go away.

(See #189 for additional discussion and background.)