[Tizen] Add signature xsd file into crosswalk

Because crosswalk has replaced WRT in Tizen common image, some metadata
file which provided by wrt-installer package are missed.
signature_schema.xsd is used to verify whether signature.xml is valid.
[Tizen] Checking widget signature

Process signature file in widget according to
http://www.w3.org/TR/widgets-digsig/#signature-verification. Because
there is no Tizen CA root in image, reference validation and signature
validation on signature will be added when CA is ready.