file.GetContentFile: stream to disk, add callback

[casperdcl]

• stop GetContentFile temporarily keeping the whole file in RAM
• add an optional callback for external progress monitoring
• test
• fixes download streaming & callbacks #28
• related improvements to file upload handling #20
• reference: https://developers.google.com/drive/api/v2/manage-downloads

And the (pydrive2) docstrings did say:

:returns: str -- utf-8 decoded content of the file

And again, it (httplib2) said:

The return value is [...] a string that contains the response entity body.

And the devs did shudder. There was wailing in the North and South. There was gnashing of teeth in the East and West.

And then one brave dev, in a quavering voice, spake thusly:

Let there be no more in-memory downloads. Let the files stream until the hard drives overfloweth. Let RAM be ignored, and let us answer the call to progress.

[casperdcl]

moving to local branch #30