Fixes xml CLI sitemap generator

[lfender6445]

issuing the following command results in an error

$ bundle exec rr --sitemap XML --progress http://www.yahoo.com
/Users/lfender_mbp/source/rubyretriever/lib/retriever/cli.rb:10:in `initialize': private method `gen_xml' called for #<Retriever::FetchSitemap:0x007fe96a178710> (NoMethodError)
    from /Users/lfender_mbp/source/rubyretriever/bin/rr:75:in `new'
    from /Users/lfender_mbp/source/rubyretriever/bin/rr:75:in `block in <top (required)>'
    from /Users/lfender_mbp/source/rubyretriever/bin/rr:58:in `each'
    from /Users/lfender_mbp/source/rubyretriever/bin/rr:58:in `<top (required)>'
    from /Users/lfender_mbp/.gem/ruby/2.1.3/bin/rr:23:in `load'
    from /Users/lfender_mbp/.gem/ruby/2.1.3/bin/rr:23:in `<main>'

make #gen_xml public and add specs - note, the specs issue a network request and slows down the suite. i didn't look into it much, is there a simpler way to stub the response?