readme updates

Added release date and fixed testimonials CPT message
Fixed release date for 3.2
Update description to use US-English

Proper US english spelling is "Customization", which is in line with where the word is used elsewhere

See: https://wordpress.org/plugins/jetpack/
Site Logo: Add backwards-compatible functions for themes using the original unprefixed template tags.

See: https://github.com/Automattic/jetpack/pull/956