Support fields in getChanges

This adds a filesFields property in getChanges method's options
parameter and also makes it honor the default fields for files
resources.
Add @const annotation to constants in GoogleDrive
Fix a bug in sendListRequest_ about fields handling

'nextPageToken' is not always included in all multi-page requests.
Add proper error handling in getAll
Add includeDeleted and includeSubscribed for getChanges

We need includedSubscribed: false to only retrieve changes made in
My Drive.
Implement trash, untrash, delete for Drive API

Also changed sendRequest to accept all 2xx status code as success
when expectedStatus is not specified. An operation property is added
to sendFilesRequest's options parameter so that trash and untrash
URLs can be used in the request.
Rename opt_options to options in download function body

The parameter was renamed but references to it were not.