Updated pictweet to use upload()

Twitter's website states that update_with_media is deprecated, so I
replaced it when their new API thingie, upload!

* My `pic_tweet` should be completely backwards compatible. It's been renamed to `pic_tweet`, but `pictweet` still works.
* Second argument now supports an array in addition to just a string. You can pass up to four filenames in this array, and it'll create a tweet with multiple pictures! We've been able to do that manually for some time now.
* You can pass a URL instead of a filename, and your bot will download it for you! You can mix in URLs and filenames in the same array too.
* You can now provide two hash option parameters as arguments 3 and 4! Argument 3 goes to the tweeting method, update, while argument 4 goes to upload. I don't think upload actually takes any options at the moment, but I implemented that just in case!
* You can attach a block to `pic_tweet`, and that block will be passed a filename for each file it plans to upload to your tweet. You can use that filename to do whatever processing you need to do! It copies local files elsewhere before passing them to this block, so you don't even need to worry about messing with them. Don't forget to .close any File objects you .open or .new in your block.
* Under very specific circumstances, tweets flagged as possibly sensitive won't show up on the website if they have multiple pictures. It's a bug with Twitter's site though, because it shows up just fine when those conditions aren't met!
* Exceptions won't be raised, and your tweet will still get posted, as long as at least one image can be uploaded to it. `pic_tweet` raises an exception of all your images failed. If you don't pass any images at all (empty array or string), it'll act exactly like tweet does. Not sure why you would do that though.
* Unlike the original `pictweet`, all File objects opened by this one are .close'd properly!
* I added a `pic_reply`, which works exactly like `reply` and `pic_tweet` does: its first parameter is a tweet or dm (untested) to reply to, and the rest of its parameters are the same as `pic_tweet`'s.
* You can provide just a file extension (like `'.jpg'` or `'.png'`) instead of a filename to create an empty file, and then you can use `pic_tweet`'s block to generate an image into it!