add support for Image creation from string image data

[troosan]

This change allows us to create an PhpOffice\PhpWord\Element\Image from a string representing the image data.

        $source = file_get_contents(__DIR__ . '/../_files/images/earth.jpg');
        $image = new Image($source);

Of course in real life $source would come from a DB for instance.

Thanks to this you do not have to write the image data to file before passing it to the constructor.
This solves issue #921

[Progi1984]

@troosan Good job in the MR. Tests units & Code are 👍 . Could you add some documentation, before the merge ? Thanks

[troosan]

@Progi1984 I updated the documentation but for whatever reason the build is failing now. The build is taking forever and finally php-doc fails to be executed.
Is there something wrong with what I committed or is there something wrong with Travis?

[Progi1984]

@troosan Perfect. PhpDoc has some bugs currently, I don't care about it in my review.