Test *real* ctor defaults.

Rather than those for testcase's '_makeOne'.
Add namespace to query ctor / clone.
Typos.
Pass namespace when fetching results.

Fixes #170.
Make '_get_protobuf_attribute_and_value' rigorous about bytes vs. text.
Make '_get_value_from_value_pb' rigorous about bytes vs. text.
Add notes to docstrings about 'text' vs. 'bytes' semantics.

Addresses #233.
Merge pull request #255 from tseaver/170-query_namespaces

Fix #170:  handle namespaces in queries
Merge pull request #263 from tseaver/233-support_blob_value

Address #233: support blob_value in protobuf
Drop munging of 's~' prefix onto keys.

Fixes #121.

Note:  may conflict with PR #257 and #258.
Merge pull request #259 from tseaver/121-drop_dataset_prefixes_in_keys

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