updated verbage for Templates for PHP>5.5
yield is in use by the new Generator functionality
updated relevant files to use yieldContent() instead
yieldContent changed to just content for more human-friendly use
also updated changelog and associated docs
Merge pull request #873 from DaveMC08/feature/fixes_php_5.5

Changed Template::yield() to Template::content() to avoid conflicts and errors in PHP 5.5+ and the new Generators.