cmd/bosun: Add epoch() func, which returns the current Unix epoch in sec...

...onds

Also required change parse.Tree.func() to support the case of a function with no arguments.

Use Case (Alert when warranty will expire):

![image](https://cloud.githubusercontent.com/assets/1692624/5416477/f7f4bd5e-81fd-11e4-9670-733670d6ddb8.png)



I think "now" might be a better name for this function. But we can stick with epoch if you feel strongly.