Remove use of XHTML to simply use the <html> tag as defined in html5
It permits to avoid check of html code, because some blogs use html and other xhtml
Revert "Fix image tags by adding a "/" at the end, because some blogs won't create html code correctly"
because we now use HTML5 instead of XHTML in our epub.

This reverts commit 0107d4a30fd2c06ad6723c6dd2dde0fcf7d9ec38.
Increment epub plugin version