Add license field to DOAP
This is a follow-up ticket of http://sourceforge.net/p/pypi/bugs/361/
The request is to add DOAP license information extracted from the categories when possible/available.
The problem is that DOAP expects a URI for the license and it turns out that http://spdx.org/licenses/ provides some.
Attached in a patch that browse the categories of a package, check if one of them is a license and if so, check if the license is known to the license table allowing mapping the category to a URI.
/!\ patch not tested (but I don't expect it breaks anything).