Enable exception handling for Windows and fixed minor variable types.

Hello,
when compiling in Windows, I received the following warning:
```C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\include\xlocale(323): warning C4530: C++ exception handler used, but unwind semantics are not enabled. Specify /EHsc [D:\midistream\node_modules\midi\build\midi.vcxproj]```

I then realised that C++ exception handling is disabled by default by node-gyp. Hence I re-enabled it again in the ```binding.gyp``` file. If you look closely, for linux and mac this is also re-enabled, so it was missing for Windows.