Remove IDisposable from NaturalSortComparer

There is no reason that NaturalSortComparer should implement
IDisposable - it doesn't deal with unamanged objects or other
objects implement IDisposable. Having it derive from IDisposable
requires us to call the Dispose() method if we want to follow
best practices.
Merge pull request #77 from ermshiperete/IDisposable

Remove IDisposable from NaturalSortComparer