Made the lib fot for dynamic loading on Windows.

* added dllexport modifiers for MSVC compilation
* included extern.h from within decode.c to expose its symbols
* deleted ud_insn_sext_imm as it isn't referenced anywhere
little readme for fellow windowsers trying to compile with VS2012
Merge pull request #61 from sgraf812/master

Some changes for loading the lib dynamically under Windows