Problems adding ESP8266 wifi board
It looks like the server is down.  Isitdownrightnow reports that it's been down for almost 4 days: http://www.isitdownrightnow.com/arduino.esp8266.com.html 

There is a discussion about it over on the ESP8266 forums: http://www.esp8266.com/viewtopic.php?f=26&amp;t=3134&amp;start=8

Let's hope it will be back again soon.