Added Twilio plugin

Hi, 

I've just finished a new uptime plugin. With the Twilio plugin you can use the (paid) Twilio API to send SMS messages of events to your mobile phone. The plugin looks much like the email plugin, but the messages are shorter and.... well it uses SMS instead of email. 

This is my first Pull Request ever, if there are things unclear to you, please send me a message. 

Cheers,
Egbert