Document further how 'text' vs. 'bytes' values are stored.

Note how the two differ in Python2 vs. Python3.

Fixes #266.
Think-o:  'bytes_value' -> 'blob_value'.

Incorporates feedback from @dhermes.
Merge pull request #331 from tseaver/266-document_text_vs_bytes_values

Fix #266: Document further how 'text' vs. 'bytes' values are stored.
Adding extra parameters for querying Cloud Storage keys.
Testing interaction with page token and extra params in iterator.

Also making sure tests pass by using the `upload_` prefix
instead of the `set_contents_` prefix.