Add license-expression library

[steven-esser]

To support #90, we should add the https://github.com/nexB/license-expression library to DeltaCode and update our algorithm to use this library.

[pratik0316]

@MaJuRG , @johnmhoran could you explain a bit more about the context of this issue , In what terms will we use this library ?

[johnmhoran]

Hi @Pratikrocks . The scan files produced by https://github.com/nexB/scancode-toolkit contain a number of fields, including license_expression. Our goal is to find a way for DeltaCode -- which compares two ScanCode-Toolkit result files -- to compare and score pairs of simplified license expression values, and we'll need the https://github.com/nexB/license-expression library to do more detailed comparisons.

[pratik0316]

Thanks @johnmhoran for the explanation