Make Message::nonce a serde_json::Value

Nonces can actually be almost anything - including booleans - so just
use a Value to represent it since very few users will need it.

This fixes errors like:
```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":12187872,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:00:00.000000+00:00","pinned":false,"nonce":"","mentions":[{"username":"redacted","id":"redacted","discriminator":"redacted","avatar":"redacted"}],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"redacted","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("Unknown i64 value: "), line: 0, column: 0 })
```

and:

```
WARN:serenity::internal::ws_impl: (╯°□°）╯︵ ┻━┻ Error decoding: {"t":"MESSAGE_CREATE","s":1001192,"op":0,"d":{"type":0,"tts":false,"timestamp":"2017-06-01T01:01:01.000000+00:00","pinned":false,"nonce":true,"mentions":[],"mention_roles":[],"mention_everyone":false,"id":"redacted","embeds":[],"edited_timestamp":null,"content":"bork","channel_id":"redacted","author":{"username":"redacted","id":"redacted","discriminator":"redacted","bot":true,"avatar":"redacted"},"attachments":[]}}
ERROR:serenity::client: Shard handler received err: Json(ErrorImpl { code: Message("invalid type: boolean `true`, expected identifier"), line: 0, column: 0 })
```

1 files changed, 3 insertions, 1 deletions

diff --git a/src/model/channel/message.rs b/src/model/channel/message.rs
index 15200fe..89973d0 100644
--- a/src/model/channel/message.rs
+++ b/src/model/channel/message.rs
@@ -1,4 +1,5 @@
 use chrono::{DateTime, FixedOffset};
+use serde_json::Value;
 use ::model::*;
 
 #[cfg(feature="cache")]
@@ -48,7 +49,8 @@ pub struct Message {
     /// Array of users mentioned in the message.
     pub mentions: Vec<User>,
     /// Non-repeating number used for ensuring message order.
-    pub nonce: Option<Snowflake>,
+    #[serde(default)]
+    pub nonce: Value,
     /// Indicator of whether the message is pinned.
     pub pinned: bool,
     /// Array of reactions performed on the message.