Block sites at specific times When at college I sometimes find myself looking at website such as Stack Exchange and Mac Rumours. Is there a way how to block sites like this on Yosemite every 1.5hr with 30min break?

What I mean is, is there an application to block specific websites at time such as 9:00-10:30 then again 11:00-12:30 etc?

This would help me to do more work at college and not get distracted. I would like an application that blocks the sites for ALL applications, however a Safari extension will work too (I only use Safari so Chrome extensions will be useless). I know editing the hosts file could work however I want it to change at certain times (I may try and make a script if no applications work :) ).
WasteNoTime is a Safari browser extension which should have the requested features according to the website:

WasteNoTime is a browser extension which is designed to help you manage your time spent on the Internet more efficiently.
Time Tracker feature gives you reports on what web sites you spent most of your time.
Instant Lockdown feature allows you to focus on your work for a period of time with limited Internet access.
Time Quota feature automatically block selected web sites when you have spent a preset amount of time on them each day.