Scheduler patch

Add 'cleanup' command

Rewrite cleanup command and take prefixed database tables into account.

Bot API 3.1

Closes #550

Make Request::encodeFile public, as it's needed to send files, letting users make use of it.

Keep track of methods that need a dummy parameter for some cURL versions to function properly.

Callbacks can be added to be executed when callback queries are called.

Introduce helper commands for all Update Entity types

This makes accessing the required Entity objects much easier.

Update changelog to include (hopefully) all changes.

Fix case when handling channel messages (request limiter)

Add custom_input for getUpdates method.

Fix indexed column length and quotes in comments.

Basic API 3.0 changes

Allow selecting channels with selectChats() and sendToActiveChats()

Use array for select-parameters, instead of individual parameters.