[Tizen] Add signature data and parser logic

SignatureFile: according to widget spec, widget should includes
signature file in the zip. SignatureFile is used to label signature
files in the widget. There are two types of signature files: author
and distributor signature.

SignatueData: it is used to store values parsing from signature.xml.
The data will be used by signature verifier to validate digital
signatures.
@rakuco , the constants are referring elements and attributes in those defined in http://www.w3.org/TR/xmldsig-core1/. Some unused are related to signature key, which will not supported in the first phase. I will remove unused constants now. When it needed, I will add them again.