Add Message::member structfield

Adds the `Message::member` structfield, which contains a partial amount
of member data (deaf and mute status, role IDs, and joined_at).

src/model/channel/message.rs

@@ -40,6 +40,9 @@ pub struct Message {
     /// message or a system message.
     #[serde(rename = "type")]
     pub kind: MessageType,
+    /// A partial amount of data about the user's member data, if this message
+    /// was sent in a guild.
+    pub member: Option<PartialMember>,
     /// Indicator of whether the message mentions everyone.
     pub mention_everyone: bool,
     /// Array of [`Role`]s' Ids mentioned in the message.

src/model/guild/member.rs

@@ -1,8 +1,8 @@
 use model::prelude::*;
 use chrono::{DateTime, FixedOffset};
 use std::fmt::{
-    Display, 
-    Formatter, 
+    Display,
+    Formatter,
     Result as FmtResult
 };
 use super::deserialize_sync_user;
@@ -462,3 +462,21 @@ impl Display for Member {
         Display::fmt(&self.user.read().mention(), f)
     }
 }
+
+/// A partial amount of data for a member.
+///
+/// This is used in [`Message`]s from [`Guild`]s.
+///
+/// [`Guild`]: struct.Guild.html
+/// [`Message`]: ../channel/struct.Message.html
+#[derive(Clone, Debug, Deserialize, Serialize)]
+pub struct PartialMember {
+    /// Indicator of whether the member can hear in voice channels.
+    pub deaf: bool,
+    /// Timestamp representing the date when the member joined.
+    pub joined_at: Option<DateTime<FixedOffset>>,
+    /// Indicator of whether the member can speak in voice channels.
+    pub mute: bool,
+    /// Vector of Ids of [`Role`]s given to the member.
+    pub roles: Vec<RoleId>,
+}

tests/resources/message_create_3.json

@@ -0,0 +1,29 @@
+{
+    "type": 0,
+    "tts": false,
+    "timestamp": "2017-01-01T01:01:01.100000+00:00",
+    "pinned": false,
+    "nonce": -6000000000000000,
+    "mentions": [],
+    "mention_roles": [],
+    "mention_everyone": false,
+    "id": "300000000000000000",
+    "embeds": [],
+    "edited_timestamp": null,
+    "content": "fake",
+    "channel_id": "100000000000000000",
+    "author": {
+      "username": "fake",
+      "id": "300000000000000000",
+      "discriminator": "1234",
+      "bot": true,
+      "avatar": "f133549aac3208319a9fbc3c12345678"
+    },
+    "member": {
+        "joined_at": "2017-11-19T18:55:33.00000+00:00",
+        "roles": [381891844067557378, 391753382181863425, 381925348956438528],
+        "deaf": false,
+        "mute": false
+    },
+    "attachments": []
+  }

tests/test_deser.rs

@@ -126,6 +126,9 @@ fn message_create() {
 
     // negative nonce
     p!(MessageCreateEvent, "message_create_2");
+
+    // message from guild with partial member data
+    p!(MessageCreateEvent, "message_create_3");
 }
 
 #[test]