implement better save macro

[JonasIsensee]

Hi,

this PR implements a new @save macro.
As far as I can see it does not break any old behaviour but it allows for more flexibility.

New features are:
Option passing:

hello = "world"
@save "test.jld2" {compress=true} hello
@save "test.jld2" {compress=true, mmaparrays=false} hello

• Assignment syntax

@save "test.jld2" bye=hello
@save "test.jld2" hello randomnumber=rand(10)

This is relevant for:
#162
closes #132
closes #133

What do you think?
Is this a good compromise in terms of syntax?

[JonasIsensee]

This PR now has an additional commit that adds iotype as a kwarg to the interface.
In this way one can now write e.g.

@save "test.jld2" {iotype=IOStream} key=val

jldopen("test.jld2", "w"; iotype=IOStream) do f
    write(f, key, val)
end

[biona001]

Hello, so what is the new syntax for saving something with compress=true?

[JonasIsensee]

Hi
the new syntax is

hello = "world"
@save "test.jld2" {compress=true} hello

but so far this is only on master.
In about 2 weeks there should be a new release so that you can update JLD2 locally for the new features.