add license information to gemspec
add license information to gemspec

This way you can get this info when using rubygems.org API

isozaki/gruff,innoq/gruff,philipgiuliani/gruff,jrmhaig/gruff,topfunky/gruff,house9/gruff,MentalPower/gruff,vigetlabs/gruff,iox/gruff,jbcden/gruff,topfunky/gruff