Add support for a license-file manifest key

This key will support projects with nonstandard licenses and the registry will
display the license as "nonstandard".

Closes #940
auto merge of #974 : alexcrichton/cargo/issue-940, r=wycats

This key will support projects with nonstandard licenses and the registry will
display the license as "nonstandard".

Closes #940
Merge remote-tracking branch 'upstream/master'