Add DEBUG() statement.

I'm adding a `DEBUG()` statement to the query language. This is a really simple feature. Just put the expression you want to print out inside the parentheses:

```
DEBUG(timestamp)
DEBUG(total_price * quantity)
DEBUG("OH YEAH!")
```

It only supports one expression right now but it's easy to add 1+ expressions. I just didn't want to spend the time right now.

**DO NOT USE THIS LIGHTLY!** If you're iterating over 10M events and doing a debug statement for each one then it's probably not going to go well for you. Use it on small datasets only.


/cc: @boourns @orenmazor @snormore