Improve support for checkLicense to allow license black listing

[ghilainm]

I have javax.mail on my classpath. It defines several licenses, it seems that defining only one license in the allowed license file is enough to pass the check but in reality, it seems wrong. It would be nice if we could have more fine-grained control in the check license task.

[nsoft]

Actually this is not usually wrong. Normally the presence of multiple licenses means you get to choose whichever one you like.

[balrok]

I opened a duplicate on #285