Add RoleSpec support for Root role

The RoleSpec class is modified slightly to add support for Root role.
Targets role is updated to match the new class definition.
Simplify Metadata generation and add default expiry

- The previous way of building Metadata files wasn't the intended way.
  Instead, creating an empty Metadata file and adding the meta data is
  more appropriate.
- The spec recommends a value of 1 day for Metadata files. This value is
  made default when creating an empty Metadata file.
Add version and default expiry to Root and Targets

- Add a version field to Root and Targets roles that defaults to 1.
  This version identified should be incremented each time a new version
  of the file is produced.
- Added the expiry values of 1 year recommended by the spec for Root and
  Targets roles.
Remove unused parameter in Verifier initializer