Display WordPress Posts: fix wrong stylesheet URL

Reported here:
* http://wordpress.org/support/topic/jetpack-creating-link-to-stylecss-in-non-existant-path?replies=1
Add changelog for 2.8
Jetpack Monitor: Fix users not connected to WordPress.com not being able to activate or configure the module.
WordPress Posts Widget: Fix an intermittent fatal error
WordPress Posts Widget: Add HTML comment to end of widget
WordPress Posts Widget: Fix PHP coding style

Also use double quotes around HTML attributes.
WordPress Posts Widget: Normalize www

When a user enters a site with a www. that doesn't exist, but the site
without the www. does exist, use the latter.
WordPress Posts Widget: Don't display more posts than exist
WordPress Posts Widget: Fix transient bugs

- `get_transient` and `set_transient` do not take a namespace parameter
- expiring transients have a name length maximum of 45 characters