Wordpress Import: removing caption tag, showing information when not importing something.

This is to solve https://github.com/ralsina/nikola/issues/188.
I found it the best to remove the wordpress ``caption`` tags because they contain redundant information that is included in the ``img`` tag aswell. I decided against transforming them into markdown (my first idea) because this would make us loose information aswell.

To make future content altering easier I did some refactoring and moved the transformation of the source code tags.

If a post contains no content nothing (not even a file with meta information) is written. This alters the previous behaviour where meta infos were written but no content file.

In this case a message is displayed. This is also the case if the download of a file fails.

Tests are included and I felt free to add myself to the authors file. ;)