Add support for uploading multiple files

[Syfaro]

This allows for uploading multiple files for media groups or thumbnails. It also fixes a longstanding bug with using URLs.

There's a few breaking changes:

• UploadFile has been renamed to UploadFiles, and now takes a []RequestFile instead of fieldname and file
• All New*Upload and New*Share have been condensed into New*
• All APIResponse and errors are returned as pointers

I'd love to get feedback on the API changes or what could be done to further improve things.

[HirbodBehnam]

Hello
Is using panic in the upload goroutine a good practice? Doesn't panics crash the whole program unless recover() is used?
If you don't want to use panic I have 3 other options you can use:

1. Use panics with recover. But also you need to change the function to func (bot *BotAPI) UploadFiles(endpoint string, params Params, files []RequestFile) (apiRes *APIResponse, err error). In the recover, set err = errors.New(fmt.Sprintf("%v", r)); So the function will return with an error. Note that you can't return values from a "defered" function. Here is an code sample.
2. Use an external error variable. For example before starting the goroutine, define a variable internalError (var internalError error). Then in the goroutine, if an error occurs, use internalError to store the error and then use return to stop the goroutine. Check the error after the line resp, err := bot.Client.Do(req). I just realized that this is a bad idea. If you do this the post request will be done (submitted to Telegram servers) without any errors and returning the error won't stop it.
3. Use PipeWriter.CloseWithError; I'm not sure if this one is a good practice or not, but you can use w.CloseWithError(err) and then simply return from the goroutine. The error will be caught at resp, err := bot.Client.Do(req)

[Syfaro]

Whoops, I forgot to mention that the panics needed to be removed still.

I wasn't aware of CloseWithError, that seems like a perfect solution, thank you!

[abserari]

waiting for that

[tjhorner]

This looks good to me, but what are we going to do about the breaking changes, since the versioning is really weird at the moment?

[tjhorner]

This smells like a breaking change to me. People running off of master or develop are going to have a bad time.

[Syfaro]

Yeah. Not really sure what else to do though, I want to keep consistency through everything and there's already a ton of breaking changes elsewhere so might as well do it now.