Add Wordpress support

Add support for Wordpress using the public Wordpress REST API. The nice thing about it is you don't need an API key.

In syte_settings.py:
1. Added a BLOG_PLATFORM parameter. Should be set to "tumblr" or "wordpress".
2. Added a WORDPRESS_BLOG_URL parameter, similar to TUMBLR_BLOG_URL.
3. Added a WORDPRESS_API_URL parameter, similar to TUMBLR_API_URL.

Changed tumblr.py to blog.py, but the Tumblr-related logic should remain unchanged.

I tested tags and offsets, but haven't checked if it works with Disqus integration.

This is my first pull request on Github, so apologies if I did anything incorrectly.