Using a fallback connection in Bucket

[dhermes]

This is an alternative fix to #728, by way of making connection be optional.

This has an effect of cascading through everything since ACL and Blob both use the connection of the Bucket.

Note especially the changes to get_bucket and create_bucket, which passes None through to the Bucket() constructor and lets the environment dictate the connection (e.g. in a batch).

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@tseaver Do you think bucket.connection being a non-deterministic value will be confounding?

[coveralls]

Coverage remained the same at 100.0% when pulling 69c321f on dhermes:bucket-connection-fallback into d34a15e on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 69c321f on dhermes:bucket-connection-fallback into d34a15e on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same at 100.0% when pulling adc470b on dhermes:bucket-connection-fallback into 4d69bb3 on GoogleCloudPlatform:master.

[dhermes]

@tseaver I rebased this after merging #760 but as mentioned, it may make more sense to just pass connection to methods that require it, as in #728 (or use the implicit fallback).

[coveralls]

Coverage remained the same at 100.0% when pulling adc470b on dhermes:bucket-connection-fallback into 4d69bb3 on GoogleCloudPlatform:master.

[dhermes]

After discussing with @tseaver it has been determined that unbundling Connection and Bucket is possible, so I am closing this out. Will salvage some parts of it, like Bucket.create() and send out another PR.