Changed highlight_phrase default tags to <mark> (from <strong>)
Added changelog note for changes to highlight_phrase()
Updated the example response for highlight_phrase()
Added a note about how to add default styling for the <mark> tag in older browsers
Merge remote-tracking branch 'ellislab/develop' into alexbilbie-highlight_phrase

Conflicts:
	user_guide_src/source/changelog.rst
Added an upgrade note
Fixed merge conflict in changelog.rst
fixed merge conflicts with html5 highlight phrase