add prefix arg to storage.bucket.get_all_keys

[proppy]

[tseaver]

The Object.list API docs support prefix as a query-string parameter, along with several others:

• delimiter (to allow for pseudo-hierachy, it looks like)
• maxResults
• pageToken
• projection
• versions

[tseaver]

Fixing #354 is a prerequisite to using prefix and delimiter to emulate hierarchy within a bucket.

[tseaver]

Rather than tweak Bucket.get_all_keys(), I plan to provide support for these extra parameters by exposing an Bucket.iterator() method, taking these parameters, and returning an instance of bucket._KeyIterator configured with them.

Needs #358 to land first.