Fixed handling of 404 errors for Rack GET and DELETE

This should resolved following issues:

https://github.com/tuskar/tuskar/issues/18
https://github.com/tuskar/tuskar/issues/22
https://github.com/tuskar/tuskar/issues/21

Note this is just an 'attempt' to fix this problem, by workarounding
the wsmext parsing and using just plain Pecan @expose().

The validation of WSME complex type input is not done when
using thing method, so this way might not be the best way for
handling PUT and POST.