Fix sort order for institution details keys

XmlHasher turns everything into strings.
Intuit's schema shows displayOrder field 
as an int. Not converting to an int causes 
'10' to appear before '2'.
Remove nokogiri from gemspec

XmlHasher is used instead of nokogiri
Remove nokogiri requires
Remove redundant calls to delete_account

Deleting a customer already deletes accounts
Clear oauth token after deleting customer

Prevents accidental reuse of token and is 
recommended by Intuit docs
Merge pull request #6 from simple10/master

Fix displayOrder sorting and remove nokogiri requirement