Sum() Finder Functionality

This is a continuation of PR #1039. (Sorry, I deleted the original forked repo.)

Adding support for using SQL's SUM() functionality. Example can be found in the sum example file `examples/sum/app.js`

Usage example:
```
Project.sum('hours', { where: { 'day': ['saturday', 'sunday']} }).success(function(hours) {
  console.log("This project took " + hours + " overtime hours to complete.")
})
```