add stream_set_blocking for objects of type stream to limit boilerplate

[steverhoades]

for creating non-blocking io.

Decided to add a PR for this after talking with @cboden on IRC. It seems like it would be beneficial to have Stream always set stream_set_blocking for all stream types.

This would eliminate the need for:
https://github.com/reactphp/socket/blob/master/src/Server.php#L46

as Connection is of type Stream and the construction occurs:
https://github.com/reactphp/socket/blob/master/src/Server.php#L69

The benefit of this is that we can now do things like this:

$stream = new \React\Stream\Stream(fopen('myfile.txt', 'r'), $loop);

[steverhoades]

Also, if this PR is accepted then #8 can be closed.