lexically sort vfields in toptable if no order is specified

On perls with hash randomization (>= 5.18) this makes UTs pass and
makes for a nicer user experience (columns don't randomly reorder themselves).
switch fromatomfeed from XML::Simple to XML::Twig

Handles large documents
Drops XML::Simple as a dependency
Merge pull request #30 from elindsey/toptable_sorting

lexically sort vfields in toptable if no order is specified
Merge pull request #31 from elindsey/fromatomfeed_twig

switch fromatomfeed from XML::Simple to XML::Twig