SitemapEntry.php optimization

* Change private properties to protected
* Add checks for setPriority and setChangeFreq
* Add constants for ChangeFreq
* return $this in setters
Debug
Cleanup

* Remove useless `use XMLWriter`
* Debug var name line 25
* Optimize setProperty method
Create SitemapEntry.php constructor

Create a constructor for quick ans simple sitemap entries
Change lastMod date format in SitemapEntry

Use W3C date format (http://www.w3.org/TR/NOTE-datetime) as it write in http://www.sitemaps.org/protocol.html#xmlTagDefinitions.
Update SitemapEntry constructor
Merge pull request #3 from nicolas-brousse/patch-1

SitemapEntry refactor