fix(#111): Implement `files` data producer

Now it's possible to declare multiple files inside the
`<data>` section. Example:

    <data>
      <type>files</type>
      <paths>
        <path>Readme.txt</path>
        <path>Changes.txt</path>
      </paths>
      <dst>/usr/share/myapp</dst>
    </data>

Note that the `dst` element is optional and used as directory if
present. Mapper could be used instead.

The mockito library is added to `pom.xml` to simplify unit testing.
Merge pull request #146 from roman-kashitsyn/multifile_datasets

fix(#111): Implement `files` data producer