Implemented the 'DefaultIfEmpty' operator. See #34

Hi,

This implements the `DefaultIfEmpty` operator from #34.

There are two variants. However, I can not get the element type in the source observable due to type erasure. So I only implemented the variant with a default value: http://msdn.microsoft.com/en-us/library/hh229624(v=vs.103).aspx

Please take a look. Thanks.