switch fromxml from XML::Simple to XML::Twig

Preserves document order (making UTs pass in perl 5.18)
Handles large documents
Fixes bug where ForceArray was always set to 1
Fixes bug where 'element' entries weren't always added when multiple --element were specified
deduplicate --elements; add argument handling test cases; minor style/documentation changes
force all nested elements into arrays
add XML related todos
Merge pull request #25 from elindsey/twig

switch fromxml from XML::Simple to XML::Twig