Add DomPDF to composer.json and enable archived image source

[ivanlanin]

Guys, please review the following changes:

• Add DomPDF to require-dev in composer.json to enable unit testing for PDF writer
• Enable zip://$archiveFile#$imageFile construct for image source to enable reading from DOCX

Thanks

[Progi1984]

@ivanlanin We will have a very good PDF writer.

Some notes :

• When the docs will be generated, warning to the table for the correct display
• Personnaly, I would add dompdf in "suggest" of composer.json with an explanation

[ivanlanin]

Thanks @Progi1984.

1. Did you mean (a) there's a warning message E_WARNING or (b) there should be a warning to users that tables are not (yet) rendered correctly with the current writers?
2. I agree, but how can we make Travis install dompdf?

[Progi1984]

@ivanlanin :

1. I mean : We should care that the rendering of the table be well.
2. For suggested packages, I think we should add travis commands in .travis.yml for install anothers packages. (Ref : https://getcomposer.org/doc/03-cli.md#require)

[ivanlanin]

1. Ok. We'll test table rendering with special care.
2. composer.json and .travis.yml updated. Travis build passed.

[Progi1984]

Perfect for me :)