New trim() method

Will remove all tabs and newlines.

Works with strings, objects, and arrays.

$content is a pointer and is updated as well as returned. Meaning:
$var = $xml->trim($var) is effectively the same as $xml->trim($var)
Class variables are protected instead of private

Also, trim $content
Resolve issue #2

Conflicts:
	resources/xml_document.php
Merge branch master
Try setting attributes on correct node in set() method

Addresses issue #3
Update attribute setting methods
Attempting to simplify __call() and set() methods.

Reduce to simplest cases.
Resolve issue #3

Now able to append nodes and attributes correctly.

If $tag is set, create a new node and update $parent, but
revert to old $parent at end.