[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.
The signature data is special data for signature XML file. If the XML file is a Tizen signature file, the XML must match structure in http://www.w3.org/TR/xmldsig-core1/. And I will firstly verify the input XML file whether match siganature.xsd, which is schema file for signature data. Only the XML is verified, it will be initialized with this structure.  Currently in the function of Initialize(),  data of signature will be parsed as schema structure.