Make gcloud.storage use a streaming HTTP library for downloads

[jgeewax]

Right now we're using httplib2 because of oauth2client working really nicely with that, however httplib2 doesn't support a streaming download.

This means, a .request() will load the entire response content into memory, which won't work at all for a big file.

To get around this, we chunk the file up using the Range HTTP header and request pieces of it until the end, however the better way to handle this is with a streaming HTTP library:

stream = <send http request>
with open('output.txt', 'wb') as output:
  data = stream.read(CHUNK_SIZE)
  while data:
    output.write(data)
    data = stream.read(CHUNK_SIZE)

To make this work we need to:

• use a library that does support streaming
• update oauth2client to work with that library

[proppy]

update oauth2client to work with that library

Or use another oauth2 client altogether.

oauthlib is a generic oauth library for python that is http client agnostic.

There is an adapter for requests: https://github.com/requests/requests-oauthlib

/cc @aliafshar

[tseaver]

Superseded by #334.