Update writeBufferRequestBacklog() parameters and add writeBufferRequestBacklogAll()

[clue]

The writeBufferRequestBacklog() method can be used to export the channel backlog (chat history). This method now uses the parameters as-is as defined by the underlying network protocol. This is a BC break with existing code, so this PR adds relevant documentation, examples and tests:

// old
$client->writeBufferRequestBacklog($bufferId, 100);

// new
$client->writeBufferRequestBacklog($bufferId, -1, -1, 100, 0);

Additionally, this PR implements the writeBufferRequestBacklogAll() method which allows you to fetch the backlog from all buffers/channels at once:

// new
$client->writeBufferRequestBacklogAll(-1, -1, 100, 0);

This PR includes a new example which shows how both methods can be used to export the chat history. This can be used as a base to build an interactive chat application or to export this to any kind of export format, such as RSS/Atom etc. (make sure to follow https://github.com/clue/quasselio for details).

Builds on top of #41