PHPDoc Tidying
Fn `Jetpack::is_development_version()`

Keys off the JETPACK__VERSION constant and flags if it’s a `-dev` or
`-beta` version or the like, or a straight-up number!
Change how we're adding the concatenated css.

Use core’s native RTL support.  Also, for stable releases, use the
Jetpack Version.  For development builds, use filemtime.