Added setContentWrapper method, to designate a section of the DOM to replace

This lets you use Turbolinks to replace a targeted part of a document rather
than always obliterating the entire body
Set 'X-Requested-With' header to 'XMLHttpRequest'

This makes it so that you can detect from Rails if a request is coming from
Turbolinks, which works nicely in tandem with the setContentWrapper option
(allowing you to conditionally serve layout-free HTML for Turbolinks requests)