Break up the model module

The `model` module has historically been one giant module re-exporting
all of the model types, which is somewhere around 100 types. This can be
a lot to look at for a new user and somewhat overwhelming, especially
with a large number of fine-grained imports from the module.

The module is now neatly split up into submodules, mostly like it has
been internally since the early versions of the library. The submodules
are:

- application
- channel
- error
- event
- gateway
- guild
- id
- invite
- misc
- permissions
- prelude
- user
- voice
- webhook

Each submodule contains types that are "owned" by the module. For
example, the `guild` submodule contains, but not limited to, Emoji,
AuditLogsEntry, Role, and Member. `channel` contains, but not limited
to, Attachment, Embed, Message, and Reaction.

Upgrade path:

Instead of glob importing the models via `use serenity::model::*;`,
instead glob import via the prelude:

```rust
use serenity::model::prelude::*;
```

Instead of importing from the root model module:

```rust
use serenity::model::{Guild, Message, OnlineStatus, Role, User};
```

instead import from the submodules like so:

```rust
use serenity::model::channel::Message;
use serenity::model::guild::{Guild, Role};
use serenity::model::user::{OnlineStatus, User};
```

1 files changed, 3 insertions, 2 deletions

diff --git a/src/framework/standard/create_command.rs b/src/framework/standard/create_command.rs
index 0df649d..6450b64 100644
--- a/src/framework/standard/create_command.rs
+++ b/src/framework/standard/create_command.rs
@@ -1,7 +1,8 @@
 pub use super::{Args, Command, CommandGroup, CommandOptions, CommandError};
 
 use client::Context;
-use model::{Message, Permissions};
+use model::channel::Message;
+use model::Permissions;
 use std::sync::Arc;
 
 pub enum FnOrCommand {
@@ -50,7 +51,7 @@ impl CreateCommand {
     ///     CommandError,
     ///     StandardFramework,
     /// };
-    /// use serenity::model::Message;
+    /// use serenity::model::channel::Message;
     /// use std::env;
     ///
     /// let token = env::var("DISCORD_TOKEN").unwrap();