new readbytes and readbytes! functions

[stevengj]

This pull request expands the functionality of the (internal) readbytes functions that we already had into two new (exported & documented) functions (inspired by the functionality in Python's RawIOBase class):

readbytes!(stream, buffer::Vector{Uint8}, nb=length(buffer))
readbytes(stream, nb=typemax(Int))

The readbytes! function reads up to nb bytes into buffer (growing it as needed), returning the number of bytes read. The readbytes function reads up to nb bytes and returns a new Vector{Uint8} buffer containing the bytes read.

(Incidentally, we had two different implementations of readall(s::IO), one replacing the other; I kept the one that uses readbytes.)

PS. I added this functionality to make it easier for PyCall to pass Julia IO objects as the corresponding types in Python.