Optionally show progress when importing remote OVA/box

Kinda useful when downloading large boxes/OVAs from remote servers:

    octoc = Fog::Compute.new( :provider => 'octocloud' )
    cube = octoc.cubes.create :name => 'test-cube',
                              :source => "https://foo.bar.com/myova.ova",
                              :extra_opts => { :show_progress => true }

When the progressbar gem is available, it will automatically use it:

    Downloading:    16% |ooooooooooooooo   |   1.2MB 331.8KB/s ETA:   0:00:19

Gracefully degrades to a simple download animation otherwise:

    Downloading... 7.68 MB
Merge pull request #15 from rubiojr/box-ova-import-progressbar

Optionally show progress when importing remote OVA/box