Support for Traditional Encryption (ZipCrypto) + Tests

[johnnyshields]

This PR adds support for traditional encryption. The interface is:

      Zip::OutputStream.write_buffer(::StringIO.new(''), Zip::TraditionalEncrypter.new('password')) do |out|
        out.put_next_entry("my_file.txt")
        out.write my_data
      end.string

This preserves backwards compatibility with existing rubyzip, however I'd suggest that it might be cleaner to do something like an options-hash based interface in future versions.

The PR contains unit and integration tests. It seems that InfoZip uses a different crc32 function from Zlib, so we had to include a zip file generated with archive-zip into the repo (couldn't get InfoZip-based generators to work.)

There is a separate PR for AES encryption (#179) which is read only. We believe our code structure is a cleaner one which allows for both read and write and it should be fairly straightforward to adapt #179 into our structure.

Big thanks to @matsu911 who did the work.

[coveralls]

Coverage increased (+1.35%) when pulling 71225dc on johnnyshields:master into deeefa1 on rubyzip:master.

[simonoff]

@johnnyshields Hi, can you look on 1.9.3 build? It failed. If you will be not possible to fix it would be better add fixes for gemfile to set required version of ruby to 2.0.0 or more.

[coveralls]

Coverage increased (+1.5%) when pulling a54191f on johnnyshields:master into deeefa1 on rubyzip:master.

[johnnyshields]

@matsu911 fixed it--thanks! Travis is now green

[johnnyshields]

@simonoff thanks for merging and @matsu911 thanks for the awesome work!

[simonoff]

@johnnyshields n/p. But would be great to update README with info how to use this feature.

[johnnyshields]

Yeah, I agree, but I think we should mark this feature as "experimental" for the time being. I personally am not 100% happy with the interface, it's harder to use than it should be. Here's what I mean:

  # current
  Zip::OutputStream.write_buffer(::StringIO.new(''), Zip::TraditionalEncrypter.new('foobar')) 

  # better (but breaks existing interface)
  Zip::OutputStream.write_buffer(password: 'foobar', encryption: :traditional)

Also, when reading a password protected zip it doesn't automatically detect the encryption type. It would be nice if @muz and @jphastings can have a look at the AES PR and chime in with their thoughts on how we can support this strategically.

[johnnyshields]

Raised PR #211 with docs

[simonoff]

@johnnyshields I think we can break API because this feature will go into 2.0.0 in any case.