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
Though... I really do intend to sink a copy of T (if T happens to be movable, and in particular cheaply movable, as all of my current implementations of these ext types are, then so much the better). 

I like to make that explicit in the interface taking T by value (Sean Parent has talked about this at length in his value semantics talks) and moving it into place. I have also examined the compiler output of GCC and Clang and for several cases where I use this type erasure idiom, the codegen is identical to the forwarded version.