Added alerting system

Added ability to send twilio page when any of multiple conditions
exist.

* alerts.php runs as cron every 5 min
* looks at tank level of a node and compares it to that node's low threshhold
* looks at the number of data point over a recent time period to see if node is up and putting out data that is within range
* the node's ideal variables are set in the constants file for that node
improved alerting system

* made messages more clear
* ability to turn off paging
new alert to check daily if alerts are off
bug fix daily alert and new twilio number
new logo and some code tweaks