Added Amazon and Paypal OAuth2 support.

Hi @pennersr, thanks for maintaining this project. Here is my Christmas present for the repo :)
Joking aside, I needed to support Amazon Login and Paypal in my project and wanted to share my implementation.

Changes:
- Add Amazon OAuth2 support.
- Add Paypal OAuth2 support.
- Fix some Spanish locale inconsistencies.
- Add Amazon & Paypal documentation.
- Change SocialToken.token length from 200 to 255. This was done to support Amazon Login and prevent the following: `DataError: value too long for type character varying(200)`

To-do:
 - Create the migration file for socialaccount.models. I couldn't figure out how to make it. 

This didn't work:
` python manage.py makemigration ... `
References:
[1] https://github.com/omab/python-social-auth/blob/master/social/backends/amazon.py
[2] http://login.amazon.com/website
[3] https://developer.paypal.com/docs/integration/direct/identity/
[4] https://github.com/paypal/rest-api-sdk-python/blob/master/paypalrestsdk/openid_connect.py