Parse and validate valid license expressions

[xjunior]

LicenseFinder should be able to parse and validate SPDX License Expressions, as they get more commonly adopted in supported packages, like RubyGems and NPM packages.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. Unfortunately, the Pivotal Tracker project is private so you may be unable to view the contents of the story.

The labels on this github issue will be updated when the story is started.

[xjunior]

Closes https://github.com/pivotal/LicenseFinder/issues/509