Changed default license to use 4.0 instead of 3.0

By default the plugin was using the old 3.0 version of the Creative
Commons BY-SA license. This has now been changed to use the new 4.0
version of the Creative Commons BY-SA license by default. NB: when
selecting a license by the user it was already using the 4.0 version of
the licenses.
Added version number & settings api validation

The plugin now has a version number which will be added to the database
upon the first save. Site settings are now also validated with the same
function as the Network settings or Personal options. In the future it
would be nice to refactor this into less code.

Fix #21
Added warning text setting

The warning text is now a setting which can be set by the siteadmin. The
text will be displayed as a warning text at the license. It is mainly
used to warn people that individual content items may have a different
license than the site's or post' license.
Fix #24 License is not shown before post anymore

Removing this code until we get more feedback if people would actually
use this instead of adding another option.
Translation files added & Dutch translation

I've added translations files for the plugin so it can be translated
using poedit or some other PO file editor. Ideally this would be
combined with Transifex, but for now this will have to do.
Fixed localization issues

Made sure all localization calls use the same localization domain and
altered the load_textdomain so it grabs the right files. Tested it with
the Dutch translation and this works flawlessly.
Fixed localization in widget

Forgot to fix the widget localization. This has been fixed now.
Prevent issues with saving options without warning

In the profile and post/page interface a warning text is not needed and
thus can be ignored.
Log with error_log only if WP_DEBUG is true

I use error_log for debugging and not just for errors. These changes
make sure the debugging is only active when WP_DEBUG is set to true.
Version bumped from 2.0-alpha to 2.0-beta

Changed version to 2.0-beta