Support Julia 0.5, enable tests on nightlies

[nalimilan]

Since you mentioned possible features only available in Julia 0.5, I gave it a try and noticed the package doesn't load. This fixes it (and the tests pass).

There are still warnings about the deprecated call syntax, but AFAICT Compat.jl does not support this yet (which is a bit surprising, but that's not essential).

[andyferris]

This is really great, thanks Milan! I was confused by this but I think I understand better now I re-read through the Julia issue.

[andyferris]

And regarding "possible features only available in Julia 0.5" these are purely speculative. :)

I'm currently doing warm-up by thinking about generic metaprogramming with @pure and I'll possibly try implement a typed-dictionary to play with interfaces. But since 0.5 isn't imminent, this is all academic at this stage.

[nalimilan]

I was mostly thinking of @pure, which would allow for a nice indexing syntax. With some support from Compat.jl, you could easily offer it on 0.5 while still supporting 0.4.

[nalimilan]

For reference, you should be able to move to the new call overloading syntax using Compat.jl soon: JuliaLang/Compat.jl#181