Support Julia 0.4 call overloading

[stevengj]

JuliaLang/julia#8712 implemented overloading of call. This will make PyCall much easier to use because we won't need our currently fragile conversion of some types into anonymous functions. Instead, we will just leave Python functions as-is as PyObject, and implement

Base.call(o::PyObject, args...; kws...) = pycall(o, PyAny, args...; kws...)

It's quite trivial to implement actually, but I'm holding off until 0.4 is closer to release because it seems like it will break PyCall into two forks (pre-0.4 and post-0.4).