[Go] Set CGO functions for Windows via DllMain

Based on the work of Wei Guangjing[1].
On Windows, dynamic binding of methods is not possible. So we use DllMain to do
that.
[1] https://groups.google.com/forum/#!topic/golang-nuts/9L0U4Q7AtyE
[2] https://github.com/swig/swig/issues/100

Closes #100

Signed-off-by: Steeve Morin <steeve.morin@gmail.com>
[Go] Mark all _wrap_ functions as SWIGEXPORT

This enables creation of DLLs on Windows.

Signed-off-by: Steeve Morin <steeve.morin@gmail.com>