fix setup for Windows

If you run the current `setup.py` file you get this error

```
Traceback (most recent call last):
  File "setup.py", line 31, in <module>
    'pygments >= 1.6.0',
  File "c:\Python27\lib\distutils\core.py", line 152, in setup
    dist.run_commands()
  File "c:\Python27\lib\distutils\dist.py", line 953, in run_commands
    self.run_command(cmd)
  File "c:\Python27\lib\distutils\dist.py", line 972, in run_command
    cmd_obj.run()
  File "c:\Python27\lib\distutils\command\install.py", line 575, in run
    self.run_command(cmd_name)
  File "c:\Python27\lib\distutils\cmd.py", line 326, in run_command
    self.distribution.run_command(command)
  File "c:\Python27\lib\distutils\dist.py", line 972, in run_command
    cmd_obj.run()
  File "c:\Python27\lib\distutils\command\install_data.py", line 58, in run
    dir = convert_path(f[0])
  File "c:\Python27\lib\distutils\util.py", line 124, in convert_path
    raise ValueError, "path '%s' cannot be absolute" % pathname
ValueError: path '/usr/share/zsh/site-functions' cannot be absolute
```

This change fixes this issue.
I check for the `os.name` since this Cygwin can still support those absolute paths. (on Cygwin `os.name` is `posix`)