Some re-arranging. Nothing significant.

### Public & Protected

I've started dropping `public` from `public method`. It's a bit
pointless and I think minimalism is the new PHP trend ;)

I've also moved everything that's public above everything that's
protected. Consumers of an API would _first_ only be interested in the
public api. Extenders of an API may have to dig deeper, and maintainers
even deeper (private).

### Streams properties

`$writeStreams` was not in use so I removed it.

`$readStreams` is not really part of the Server's object state. It makes more sense to manually pass this array to the methods that need them. As you can see, it's only needed by a single method.

This actually makes everything a bit simpler, because there's 1 less array you need to worry about :)

### stream_select timeout

200.000 waiting time in stream_select is excessive. This loops anyway, so there no reason to use a very low amount. I removed this property as it's something that's not yet needed to be controlled by a user.

### StreamSelectFailed

This exception is actually trigged in normal usage, when there's no sockets where anything happened. We shouldn't treat this as a fail condition.

Removed the exception. We can re-create it if we need it for something else.

### Introducing 'tick'

I added a 'tick' function which does what stream_select did. This is how you can write your unittests in PHPUnit ;)