Faster, more stable image commands

Images commands stability improved and other things about the bot:

+ I completely rewritten image cog (module), and made a good cleanup of useless code (almost 600 lines (yes lul don't judge me)), now all images commands are more faster than before and errors are handled correctly. For Reddit images I am now using the same logic of my Nsfw cog.

+ All images commands where images are coming from reddit (,art ,cute ,meme ,photo ,wallpaper and ,subreddit) are now more random instead of before. So you will no longer receive the same image several times in a row which was really annoying.

+ A message is now sent if an API is down or something else, instead of making an error.

+ Gfycat links now works correctly.

+ New command: ,ferret to send ferrets images.

Now the next things will be in nsfw, streams and trivia commands. There's some things I need to fix and change.

And then I will update to Red 3.1.2 (or 3.1.3 in the meantime) (Red is the bot on which Martine is based for those who don't understand, so Martine is a fork of this bot, look the bot website for more info), which will change some things since the bot will use stable version of discord.py, and cause a little downtime while updating, but I will make an announcement in due time about that.