Use chrono for struct timestamp fields

Chrono is easier to use than timestamped strings, so they should be
automatically deserialized and available for the user, instead of having
the user deserialize the strings themselves.

These fields have been changed to use a type of `DateTime<FixedOffset>`:

- `ChannelPinsUpdateEvent.last_pin_timestamp`
- `Group.last_pin_timestamp`
- `Guild.joined_at`
- `GuildChannel.last_pin_timestamp`
- `Invite.created_at`
- `Member.joined_at`
- `Message.edited_timestamp
- `Message.timestamp`
- `MessageUpdateEvent.edited_timestamp`
- `MessageUpdateEvent.timestamp`
- `PrivateChannel.last_pin_timestamp`

`Member.joined_at` is now also an `Option`. Previously, if a Guild
Member Update was received for a member not in the cache, a new Member
would be instantiated with a default String value. This is incorrect
behaviour, and has now been replaced with being set to `None` in that
case.

Id methods' `created_at()` method now return a `chrono::NaiveDateTime`
instead of a `time::Timespec`, and `User::created_at` has been updated
to reflect that.

Additionally, drop `time` as a direct dependency and use chrono for
internals.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/model/invite.rs b/src/model/invite.rs
index 7b67296..7ff3fd1 100644
--- a/src/model/invite.rs
+++ b/src/model/invite.rs
@@ -1,3 +1,4 @@
+use chrono::{DateTime, FixedOffset};
 use super::*;
 
 #[cfg(feature="cache")]
@@ -228,7 +229,7 @@ pub struct RichInvite {
     /// The unique code for the invite.
     pub code: String,
     /// When the invite was created.
-    pub created_at: String,
+    pub created_at: DateTime<FixedOffset>,
     /// A representation of the minimal amount of information needed about the
     /// guild being invited to.
     pub guild: InviteGuild,
@@ -302,7 +303,7 @@ impl RichInvite {
     /// #         name: "foo".to_owned(),
     /// #         kind: ChannelType::Text,
     /// #     },
-    /// #     created_at: "bar".to_owned(),
+    /// #     created_at: "2017-01-29T15:35:17.136000+00:00".parse().unwrap(),
     /// #     guild: InviteGuild {
     /// #         id: GuildId(2),
     /// #         icon: None,