Add auth.service_account()

[burnash]

Simplify auth with service accounts. This PR adds gspread.service_account() which wraps Credentials.from_service_account_file cutting down initialization code.

So instead of

import gspread
from google.oauth2.service_account import Credentials

scopes = ['https://spreadsheets.google.com/feeds',
         'https://www.googleapis.com/auth/drive']

credentials = Credentials.from_service_account_file(
    'path/to/the/downloaded/file.json', 
    scopes=scopes
)

gc = gspread.authorize(credentials)

You put your service account key in ~/.config/gspread/service_account.json and service_account() does the rest:

import gspread
gc = gspread.service_account()

[honzajavorek]

Awesome!