Fixed Windows compilation errors and memory leak

Fixed Windows compilation errors introduced by https://github.com/zeromq/aziomq/pull/8
and memory leak discussed in https://github.com/zeromq/aziomq/pull/9#discussion-diff-20735727
The rationale is that otherwise this ctor will be used instead of copy ctor when ``socket_ex`` non-const reference is passed to ctor. See more here: http://ericniebler.com/2013/08/07/universal-references-and-the-copy-constructo/, look for *Special Problems with the Special Member Functions*