add ability to set request headers

This adds the ability to set HTTP request headers for download tasks.

For example:

    download {
        src 'http://download.oracle.com/otn-pub/java/jdk/8-b132/jdk-8-macosx-x64.dmg'
        dest buildDir
        header 'Cookie', 'oraclelicense=accept-securebackup-cookie'
    }