Fixed widget to work with the new plugin code

The widget has barely changed, for now and will call the new function in
the plugin code to display the license.
First attempt to restore displaying the license

The code for getting the right license needs more testing, because I
have the feeling this is not the best way to select a license. Also need
to look at pages where there's more than one post (archives, search
results, home page etc) and how to deal with the differences in
licenses (especially considering the widget which implies one licens for
all).
Include locale in license chooser

Using the lang parameter in the call for the CC partner url we can
change the license chooser's interface language. The language is grabbed
from the get_locale() function in WordPress and should work even if the
locale is not supported by the license chooser since it default to en_US
automagically. Fixed #12
Merge branch 'feature-widget' into feature-refactor