Even though it's common to use a PLUGINNAME_URL constant for creating file paths, the use of this breaks the plugins_url filter.

This pull request replaces all uses of the WPSEO_URL constant with the correct usage of plugins_url()
Removed WPSEO_URL definition
Making this change isolated and updated readme
Removed unused WPSEO_FRONT_URL constant
Merge branch 'master' of git://github.com/jdevalk/wordpress-seo into Fix-breaking-of-plugins-url-filter-hook

Conflicts:
	readme.txt
Redo MP6 fix as auto-merge had undone it
Merge pull request #312 from jrfnl/Fix-breaking-of-plugins-url-filter-hook

Fix breaking of plugins url filter hook