Allow bozo to include external/extra libraries in a nuget library package

To include external libraries that may not have the same name as the project, you can include this in `bozorc.rb` 
     
    p.library 'Zopa.Project1', 'SomeAPI.dll'

Or to include a few extra libraries

    p.library 'Zopa.Project1', 'SomeAPI.dll', 'AnotherLib.dll'

This will make the `nuspec` file generated to include the libraries as additional files.