Collect and attach NOTICEs content to the packages

[pombredanne]

It would be useful to attach the detected notice text content to a package record if it does not have one.

This could mean:

• using the content of a NOTICE top level/key file
• using the content of detected notices (as opposed to license text) in other top level/key files such as a README
• using aggregated unique/most common file-level notices of the other package files.