Added support for diffs (Issue #3)

# Diffs

The driver supports Factual's Diffs feature, which enables Factual data
update downloads.

## Simple Diffs Example

The <tt>Fetch</tt> method gives the diff data:

// Request all diffs from the US Places dataset that were generated in a
for window of just over 24 minutes, on Fri, 07 Dec 2012 13:41:03 -0800
DiffsQuery diffs = new DiffsQuery()
.After(1354916463822)
.Before(1354917903834);
var response = Factual.Fetch("places-us", diffs);