Multiple errors on Windows

[tushev]

With the following code

geoip = GeoIP2Fast()
response = geoip.lookup(ip_address)

on Windows, there is always an error in console like:

ps: unknown option -- o
Try `ps --help' for more information.

I believe this is due to this line:

geoip2fast/geoip2fast/geoip2fast.py

Line 140 in c923653

result = subprocess.check_output(['ps', '-p', str(os.getpid()), '-o', 'rss='])

Could you please fix this by checking what OS is actually running?
So, when we're on Windows, it's better not to invoke ps PowerShell interpreter without need...

Thanks!