Introduce asyncForEach utility function

It will be useful in many cases since a lot of operations are
asynchronous.
Improve getChildren to support search query
Move sendRequest out of GoogleDrive

sendRequest is used to send a generic XHR. It's now moved to
the new RequestSender class and token related code is moved into
GoogleDrive's sendDriveRequest_ method.
Add options to most GoogleDrive methods and other cleanups

Most public methods in GoogleDrive now has an options parameter
so that fields and other additional parameters can be specified.
Also add missing documentation for public methods. Rename some
private methods with a trailing underscore.

Note that those new options parameters are not yet functioning.
Add support for fields in all methods except upload/update

Now almost all methods support fields option. Upload related methods
will be modified later to support this.