Added 'AttrToSymbol' option; other changes

AttrToSymbol option, renamed xmlsimple.rb to xml-simple.rb (to match gem
name), and updated documentation.
Adjusted version number and date in gem spec.
Adjusted version information.
Renamed xml-simple.rb back to xmlsimple.rb.
Removed an entry, because it was not longer valid.
Reverted renaming.