Git Importer: add setter for commit and tag

Example:

 only_in_flavor 'master' do
    setup_package 'external/opencv' do |pkg|
        pkg.importer.branch = nil
        pkg.importer.commit = nil
        pkg.importer.tag = '3.0.0-alpha'
    end
end
Merge pull request #4 from D-Alex/master

Git Importer: add setter for commit and tag