Make closing a FileSystem also close all associated resources (streams, channels, watch services, etc.) and cause almost all other methods to throw ClosedFileSystemException, as per the spec of FileSystem.close(). Does not make Path methods that do not involve the file system itself throw, even if that would technically be the correct thing to do... just doesn't seem worth it to me.

Fixes Github issues #3 and #4.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=67288036
Update Truth dependency to 0.16.
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=67288920