Add SheetsV4::GoogleExtensions

[jcouball]

The SheetsV4::GoogleExtensions module provides extensions to the Google::Apis::SheetsV4
classes to simplify use of the SheetsV4 API.

These extensions are not loaded by default and are not required to use other parts
of this Gem. To enable these extension, you must:

require 'sheets_v4/google_extensions'

SheetsService Extensions

Functionality is added to get_spreadsheet to set the sheets_service attribute on
the returned spreadsheet and set the sheets_service and spreadsheet attributes
on the sheets contained in the spreadsheet.

This can simplify complex spreadsheet updates because you won't have to pass a
sheets_service, spreadsheet, and sheet objects separately.

Spreadsheet Extensions

The sheets_service attribute is added and is set by SheetsService#get_spreadsheet.

Sheet Extensions

The sheets_service and spreadsheet attributes are added. Both are set when the
sheet's spreadsheet is loaded by SheetsService#get_spreadsheet.