Make all 'write' methods' callbacks optional

If the caller doesn't care about the result of the operation, it
can omit the callback parameter.
Remove GoogleDriveEntry

It doesn't seem to fit well in the current code. Remove it for now.
Fix a bug in drive.js where getAll should return an array

sendListRequest_ was modified to pass the response object instead of
only items because largestChangeId is an important field. However,
the response of files.list only contains items.
Fix GoogleDrive.download to use sendDriveRequest_
Remove custom prettyPrint parameter and never pretty print
Support JSON request body in RequestSender.sendReqeust

If the request body passed to RequestSender.sendRequest is an
Object, it will be stringified before sending and content type
will be automatically set to 'application/json'.

JSON response body is not supported, unfortunately.
Explain the subtle behavior of deleting a folder discovered in test