Edited nag boxes when API requests are 90% of max.
Removed the 80% API usage email notice.
Changed the text in the API usage emails.
Converted emails sent from plain text to HTML.

git-svn-id: https://plugins.svn.wordpress.org/datafeedr-api/trunk@871934 b8457f37-d9ea-0310-8a92-e5e31aec5664