Add dataTXT and allow match score

This pull request come with two updates:

1. add dataTXT as a service, which is a tool developed by SpazioDati that performs named entity extraction on both English and Italian texts, linking to dbpedia;
1. allow to set the match score, to be used in refine for better data handling; this is implemented through the following changes:
  * NamedEntityMatch is a new class that is basically a triple: (label, URI, score);
  * NamedEntity now contains only a label and a list of NamedEntityMatch instances.

  The label of the named entity is used as the cell value, while the label of the named entity match is used as the reconciliation text. For example, in the text `I love linkeddata` we would have a structure as follows:
  ```
    NamedEntity(
      label="linkeddata"
      matches=[
        NamedEntityMatch(
          label="Linked data",
          URI="http://dbpedia.org/resource/Linked_data",
          score=0.5
        )
      ]
    )
   ```
   This allows more flexibility as it keeps the label as it was in the original text, and stores the matched label in the recon object leaving refine to handle it.