support for two-legged OAuth

[idoshilon]

Add support so scrapy will support two-legged OAuth , similar to the work of - https://github.com/simplegeo/python-oauth2

[juanriaza]

Maybe using https://github.com/idan/oauthlib

See #600

[bartaelterman]

Certainly now Google considers the authentication currently implemented in Scrapy as not using modern security standards and will block authentication attempts by Scrapy. You can Allow less secure apps to the gmail account you're sending emails from, but that's not a desired situation.