Cleanup loaders
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/FluentDOM/Loader/Html.php
	src/FluentDOM/Loader/HtmlFile.php
	src/FluentDOM/Loader/HtmlString.php
	src/FluentDOM/Loader/Xml.php
Let isNodeOffset() return FALSE for attribute offset, throw exception for invalid offset