start_reading in read{T}(::AsyncStream, ::Array{T, N})

The code snippet below is a simple `echo server`, which demonstrating a bug of `read{T}(::AsyncStream, ::Array{T, N})`  :

```
server = listen(Base.IPv4(0,0,0,0),uint16(9000))
client = Base.wait_accept(server)
p = Array(Uint8, 6)
read(client, p) ##### BUG: hangs here due to reading is not started
write(client, p)
```
And, this commit fix this :)