Gettext.cs: 
* Method 'GetPluralString' had several flaws, esp. it did not work correctly with negative numbers, and in case the Resource Manager was initialised with the given language 'treatZeroAsPlural = false' was ignored. Also, Exception handling was missing.
* Added read-only Property 'ResourceManagerInitialisedWithGivenLanguage'.

test.cs: added a NUnit Test for Method 'GetPluralString' that ensures full Code Coverage and also ensures that 'GetPluralString' works correctly when the Resource Manager was initialised with the given language.