Add support for version v2 for Google signed URLs

[databaaz]

The current v4 version used for generating signed URLs has a max expiration time of 7 days, any value set higher than this for GS_EXPIRATION leads to an exception.

I suggest to allow the users to set an additional flag (such as GS_SIGNED_URL_VERSION) to choose which one they prefer to use. Would greatly help for use cases where a longer expiration time is desired.

[jschneier]

This is fixed by #1193.