Making Blob.exists() work as Bucket.exists().

[dhermes]

Also returning the _BucketIterator directly in
storage.api.list_buckets() rather than wrapping it in iter().

@tseaver As mentioned #812 (comment) I said I'd fix Blob.exists() while making sure the iterators didn't work in batches.

It turns out they were already not working. Here is a working get request

>>> from gcloud import _helpers
>>> from gcloud import storage
>>> 
>>> _helpers._PROJECT_ENV_VAR_NAME = 'GCLOUD_TESTS_PROJECT_ID'
>>> storage.set_defaults()
>>>
>>> with storage.Batch() as batch:
...     bucket = storage.get_bucket('bucket-name')
...     print bucket._properties
... 
<gcloud.storage.batch._FutureDict object at 0x7f7ce19f4a10>
>>> bucket._properties
{u'kind': u'storage#bucket', u'name': u'bucket-name', ...

while a bucket iterator fails since _FutureDict.get throws a KeyError:

>>> with storage.Batch() as batch:
...     buckets_iter = storage.list_buckets()
...     response = buckets_iter.get_next_page_response()
... 
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "gcloud/storage/iterator.py", line 115, in get_next_page_response
    self.next_page_token = response.get('nextPageToken')
  File "gcloud/storage/batch.py", line 96, in get
    key, default))
KeyError: "Cannot get('nextPageToken', default=None) on a future"

and a blob iterator fails (but not quite always)

>>> bucket.connection
<gcloud.storage.batch.Batch object at 0x7f3b1c56db50>
>>> bucket.connection._connection
<gcloud.storage.connection.Connection object at 0x7f3b1c56d490>
>>> bucket._connection = None
>>>
>>> with storage.Batch() as batch:
...     blobs_iter = bucket.list_blobs()
...     response = blobs_iter.get_next_page_response()
...
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
  File "gcloud/storage/iterator.py", line 111, in get_next_page_response
    response = self.connection.api_request(
AttributeError: 'NoneType' object has no attribute 'api_request'

[coveralls]

Coverage remained the same at 100.0% when pulling bf8b655 on dhermes:fix-blob-exists into aafcd2a on GoogleCloudPlatform:master.

[tseaver]

LGTM