Implement Gigasecond exercise

Adding the [Gigasecond
exercise](http://x.exercism.io/problems/gigasecond) for the Rust track.

This exercise is a little odd in Rust as the core language has no library for
dealing with time. There was the Time crate, but that was pulled from
the standard library. Based on [this
RFC](rust-lang/rfcs#619) from last year, the
Chrono crate appears to be the best current library for dealing with
time. I've used Chrono here.

Either because I'm still new to Rust, or because of an actual limitation
in Chrono, the best way I could write the tests was to have everything
be a UTC DateTime. This means that the tests that only use dates still
require the slightly weird `.and_hms(0,0,0)` function at the end to make
them DateTime-y.

I did this because Chrono only seems to support Duration addition for
DateTimes, not Dates.

Author: whit0694

exercises/gigasecond/Cargo.lock

@@ -0,0 +1,7 @@
+[package]
+name = "gigasecond"
+version = "0.0.0"
+
+[dependencies]
+chrono = "0.2"
+

exercises/gigasecond/Cargo.toml

@@ -0,0 +1,6 @@
+extern crate chrono;
+use chrono::*;
+
+pub fn after(start: DateTime<UTC>) -> DateTime<UTC> {
+    start + Duration::seconds(1_000_000_000)
+}

exercises/gigasecond/example.rs

@@ -0,0 +1,39 @@
+extern crate gigasecond;
+
+//Note: Your code must also start with the following two lines.
+extern crate chrono;
+use chrono::*;
+
+#[test]
+fn test_date() {
+    let start_date = UTC.ymd(2011, 4, 25).and_hms(0,0,0);
+    assert_eq!(gigasecond::after(start_date), UTC.ymd(2043, 1, 1).and_hms(1,46,40));
+}
+
+#[test]
+#[ignore]
+fn test_another_date() {
+    let start_date = UTC.ymd(1977, 6, 13).and_hms(0,0,0);
+    assert_eq!(gigasecond::after(start_date), UTC.ymd(2009, 2, 19).and_hms(1,46,40));
+}
+
+#[test]
+#[ignore]
+fn test_third_date() {
+    let start_date = UTC.ymd(1959, 7, 19).and_hms(0,0,0);
+    assert_eq!(gigasecond::after(start_date), UTC.ymd(1991, 3, 27).and_hms(1,46,40));
+}
+
+#[test]
+#[ignore]
+fn test_datetime() {
+    let start_date = UTC.ymd(2015, 1, 24).and_hms(22,0,0);
+    assert_eq!(gigasecond::after(start_date), UTC.ymd(2046, 10, 2).and_hms(23,46,40));
+}
+
+#[test]
+#[ignore]
+fn test_another_datetime() {
+    let start_date = UTC.ymd(2015, 1, 24).and_hms(23,59,59);
+    assert_eq!(gigasecond::after(start_date), UTC.ymd(2046, 10, 3).and_hms(1,46,39));
+}