Clarify "the html element" for progress bars

When I first read "the progress bar is implemented on the html element" I thought, "What html element?" I think this change makes it a bit clearer that it's the actually the `<html>` tag
Merge pull request #417 from smidwap/patch-1

Clarify "the html element" for progress bars
Note that the progress bar will be on by default from Turbolinks 3.0
Release new version with the progress bar feature
Reflect the redirected url before actually changing the page, so page:change listeners get to see the final URL, rather than the before-redirect URL
Bump for bug fix
remove duplicate word from README.
Added link to Grails Turbolinks Plugin
Give progress bar a high z-index value by default
Use document.documentElement.cloneNode() when parsing new documents

This lets browsers correctly choose which image to use when using
resolution independent html features such as:

    <img src="image.png" srcset="image@2x.png 2x, image@3x.png 3x">

Without this fix, in many browsers pages loaded through turbolinks
will never show the higher resolution alternatives even when viewed
on retina-like displays.

The reason for this is that browsers based on the latest versions
of both Blink and WebKit (and maybe other engines too) set the
device pixel ratio (DPR) of detached DocumentElements to 1.0. They
only gain a correct DPR when displayed in a browser window, by which
time the choice between multiple resolution images has already been
made.

This affects all existing document creation methods, as they all create
new DocumentElements. Using cloneNode() instead on the existing document
creates a new DocumentElement that inherits its DPR from the currently
displayed page, thereby allowing the correct images to be chosen.
Merge pull request #423 from mattbrictson/patch-1

Give progress bar a high z-index value by default
Fix back button broken on redirect
Merge pull request #430 from phallguy/master

Fix back button broken on redirect
Merge pull request #425 from tomafro/use-clone-node-to-enable-resolution-independence

Use document.documentElement.cloneNode() when parsing to fix resolution independence
Guard against enabling the progress bar in unsupported browsers

Fix #429
Fix #426
Merge pull request #431 from javan/fix-progress-bar-when-unsupported

Guard against enabling the progress bar in unsupported browsers
Bump version and catchup the CHANGELOG
Merge pull request #421 from yellowsnow/patch-1

Added link to Grails Turbolinks Plugin
Merge pull request #420 from jtperreault/master

remove duplicate word from README.
keepscroll position fix