API Adding setURL to HTTPRequest object

The current RootURLController needs to be able to change the url of a
request, so I've added it.
FIX: Prevent opening preview when clicking the link for current page again
Update rich-text-editing.md

`HtmlEditorField::add_extension('MyToolbarExtension');` was not working for me. I had to use `HtmlEditorField_Toolbar`. 
BUGFIX: Removed XHTML XML declaration requirement
Merge pull request #1256 from g4b0/xhtml-xml-schema-not-required-31

BUGFIX: Removed XHTML XML declaration requirement
ENHANCEMENT Allow multiline comments in SS3 templates
Merge pull request #1249 from ryanwachtl/patch-1

Update rich-text-editing.md
Merge pull request #1248 from kinglozzer/leftandmain_menu_preview_fix

FIX: Prevent opening preview when clicking the link for current page again
Merge pull request #1242 from dhensby/request-patch

API Adding setURL to HTTPRequest object