Allow sending files by uploading them in SendMediaGroup

[unreal4u]

This ticket is related to #75 .

Right now, it's only possible to send files already uploaded on the Telegram servers or by passing an URL so that Telegram can download it.

It is however, not possible yet to upload several files in one go trough SendMediaGroup, which should however be possible.

Investigate how to enable and implement this.

[unreal4u]

Note: this doesn't work for either v2 and v3

[unreal4u]

PR #95 did the photos part, I'll pick this up and integrate videos as well.

[unreal4u]

So after a pretty big refactor, I came up with a solution I'm finally happy with to deal with this issue. For an example of how this works now, please refer to the example file: https://github.com/unreal4u/telegram-api/blob/master/examples/send-media-group.php .

I'll be creating a new release soon which will include a lot of updates as well.