Fix pandoc couldn't find on Windows issue.

On Windows, this plugin always cannot find pandoc executable in PATH even pandoc.exe is in PATH environment. The only way I can make it work is set pandoc-path explicitly which is painful because I use sublime text in multiple machines and platforms.

I think it's a bug. In Pandoc.py line 95, _find_binary was called without extension name (.exe) as below:

_find_binary('pandoc', _s('pandoc-path'))
On windows, this function should be called as `_find_binary('pandoc.exe', _s('pandoc-path'))'.

Thanks,
Crane