Fix for reported filesize error and numerous other small changes - also download error!!

Summary of my changes:

#### Fixes / new feature
* Fixed issue with internal domains not being properly set + resulting issue which caused an error when retrieving file sizes for certain file/domain combinations.
* Added internal variable for additional debugging
* Added various debugging snippets for advanced debugging of path / file size retrieval issues

#### Other (small) changes
* Removed some left-over superfluous code
* Removed a superfluous text string from settings page
* Fixed mini typo and some html perfectionism ;-)
* Changed version numbers to be ready for the 3.1(.0) release
* Dotted the i's and crossed the t's in the readme file
* Added a section on error reporting to the readme

#### js
* Some small code style consistency changes in .js file + regenerated min.js file to reflect these

#### Localization
* Regenerated .pot file for text changes + it was missing the notes to translators
* Finished Dutch translation

#### Banner/screenshots in new assets directory
* Regenerated admin screen screenshot + added one for the advanced settings area
* Created banner for the plugins page. I'm not much of a graphics wiz, still I hope you like it and it's better than not having one ;-)

To get this working, at the top level of the plugin's SVN repository, a new directory should be added, called 'assets'. The banner should be placed there.
Also, the screenshots can be moved from the trunk to the new assets directory. This way the download for users will be smaller, while the screenshots will still be available.

If you like and if you feel comfortable with this, add me as a committer to the WP SVN for the plugin and I'll set this up.

# **Critical issue**
I also noticed that currently when people click the download button in the repository, they will get a 404 error.
This has something to do with how the SVN repo is set up, trunk versus tags. If you like, I can have a look at it and release the 3.1 version while I'm at it.

Smile,
Juliette