Add earthquakes script

This is just a simple hubot script to pull the latest earthquakes recorded in http://earthquake.usgs.gov.

Usage:

`hubot quakes all week 5` which would pull up the last 5 quakes of the week

The intensity can either be:
`1.0`, `2.5`, `significant` or `all`

The period can be:
`hour`, `day`, `week` or `month`

The limit is an optional numeric value.

The output looks something like this:
`Magnitude: 1.9, Location: 16km NE of Hawaiian Ocean View, Hawaii, Time: 22:19:37 - http://earthquake.usgs.gov/earthquakes/eventpage/hv60388891`