Use webmock for testing.
Add shoulda test dependency.

This was originally added in d8a950668878de619aebb30e7897a03d5c12716b and then
removed when the gemspec was regenerated in
227e88098e9eb3bb4164d737585825a256039db8.

shoulda is required when running `rake test`; it simply needs to wind up in
`Gemfile` either directly or indirectly via the gemspec.
Move require to base class.

The missing require for rubyzip was preventing the tests from being run from the
command line.

This is because rubyzip has a slightly weird api.

  [1] pry(main)> require 'zip/zip'
  true
  [2] pry(main)> Zip::ZipFile.instance_methods.include? :file
  false
  [4] pry(main)> require 'zip/zipfilesystem'
  true
  [5] pry(main)> Zip::ZipFile.instance_methods.include? :file
  true

The files defining the subclasses of GenericSpreadsheet were requiring
`zip/zipfilesystem`, but `excel.rb` omitted this.
Able to open URIs with query strings.
Merge pull request #6 from hjdivad/uri-with-querystring

Open URIs with query strings.