Resolving cyclic imports in storage package.

[dhermes]

This is accomplished by moving the definitions of
BucketIterator and KeyIterator into their respective
modules (i.e. storage.bucket and storage.key).

This is not so big an issue because the parent class
iterator.Iterator does most of the work and the children
only require a small tweak.

Partially addresses #260 and is the first commit from #267

[tseaver]

LGTM, pending Travis.

[coveralls]

Coverage decreased (-0.02%) when pulling 211a3e6 on dhermes:fix-cyclic-imports-storage into 99e4d53 on GoogleCloudPlatform:master.

[tseaver]

Hmm, coveralls has coverage dropping on one line. The '_Bucket' testing jig class is no longer used.

[tseaver]

Looks like 'gcloud.storage.iterator.KeyDataIterator' should move to 'gcloud.storage.key' as well: Key is its only client.

[dhermes]

Removed the unused _Bucket class.

I intentionally didn't move KeyDataIterator because

• it was a non-trivial class (unlike the two moved)
• it is actually an iterator and hence belongs in the iterator module (unless we decide to remove this module)

[coveralls]

Coverage remained the same when pulling 8de933f on dhermes:fix-cyclic-imports-storage into 88b0ec7 on GoogleCloudPlatform:master.

[coveralls]

Coverage remained the same when pulling 8de933f on dhermes:fix-cyclic-imports-storage into 88b0ec7 on GoogleCloudPlatform:master.

[tseaver]

LGTM.