Enforce linting in Travis (not as 'after_success').

Closes #178.
Resolving cyclic imports in storage package.

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.
Removed unused _Bucket class from iterator test.
Merge pull request #270 from tseaver/178-enforce_lint_in_travis

Enforce linting in Travis (not as 'after_success').
Merge pull request #269 from dhermes/fix-cyclic-imports-storage

Resolving cyclic imports in storage package.
Revert "Fix #121:  Drop munging of 's~' prefix onto keys."
Merge pull request #274 from GoogleCloudPlatform/revert-259-121-drop_dataset_prefixes_in_keys

Revert "Fix #121:  Drop munging of 's~' prefix onto keys."