Integrate license matches and normalization via SPDX.

This allows all license matches to have a known, standardized license URL.
Also adjust pattern signatures to align with SPDX licenses, removing a few
that seem overly weak and now unnecessary.
Update Readme and tests.
Add support for overrides.

See Readme.
Minor tweaks to Readme and code style/formatting.
Merge pull request #5 from jlevy/spdx

Spdx and overrides