[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.
within Xwalk we'll work only with SignatureData (after it has been initialized), right? No need to be aware of XML details. This class should be split to a plain data class and a parser, used to initialize the data class.