contrib_bots: Restructure bots to follow a consistent structure.

Now all the bots that are stored in contrib_bots are in the
same file/directory format.
The format is specified here #3427. Add tests.py file for encrypt_bot as well.
Fixes #3427.

contrib_bots/bots/xkcd/readme.md

@@ -12,29 +12,29 @@ xkcd bot has four commands:
 1. `help`  
 This command is used to list all commands that can be used with this bot.
 You can use this command by typing `@xkcd help` in a stream.  
-![](xkcd-help.png)
+![](assets/xkcd-help.png)
 
 2. `latest`  
 This command is used to fetch the latest comic strip from xkcd. You can use
 this command by typing `@xkcd latest` in a stream.  
-![](xkcd-latest.png)
+![](assets/xkcd-latest.png)
 
 3. `random`  
 This command is used to fetch a random comic strip from xkcd. You can use
 this command by typing `@xkcd random` in a stream, xkcd bot will post a
 random xkcd comic strip.  
-![](xkcd-random.png)
+![](assets/xkcd-random.png)
 
 4. `<comic_id>`  
 To fetch a comic strip based on id, you can directly use `@xkcd <comic_id>`,
 for example if you want to fetch a comic strip with id 1234, you can type
 `@xkcd 1234`, xkcd bot will post a comic strip with id 1234.  
-![](xkcd-specific-id.png)  
+![](assets/xkcd-specific-id.png)  
 
 If you type a wrong command to xkcd bot, xkcd bot will post information
 you'd get from `@xkcd help`.  
-![](xkcd-wrong-command.png)
+![](assets/xkcd-wrong-command.png)
 
 And if you type a wrong id, xkcd bot will post a message that an xkcd comic
 strip with that id is not available.  
-![](xkcd-wrong-id.png)
+![](assets/xkcd-wrong-id.png)