Make Contexts context managers

[bgrant]

Based on #573, for convenience. Closes #558.

[kwmsmith]

I'm on the fence about this one. Given that Context.close() is a no-op for an MPIContext, and we don't want to encourage the creation of many Contexts given their overhead, I'd prefer to wait until the need for this is clear.

[bgrant]

So we're encouraging reusing Contexts instead of closing them?

Also, were you thinking of a new name for them?

[kwmsmith]

Closing for now, based on discussions. Can re-evaluate later.