Added detection of win64, and added Program Files (x86) as the location ...

Why: The firefox addon didn't work out of the box because my firefox is not located in C:\Program Files, but rather C:\Program Files (x86). This is a common problem for those running 64-bit windows.

What: Propose adding a win64 entry in the default binaries object, and additional logic in the _getDefaultBinary function to determine if we should use win32 or win64 paths.

How: Checking the platform is win32 and that the process.arc === "x64" should be sufficient in determining the user is on 64-bit windows.

I'm not sure what to do with the browserTypes object that seems to have win32 entries in it. I couldn't find anywhere those were used so I left them untouched. I would imagine we want these to have x64 equivalents too.