add support of php attributes

[eric-therond]

fix #90

[eric-therond]

@nikic or @ircmaxell do you have time to review it?

I also plan to create a separate PR for the support of arrow function and attributes on arrow function.

[ircmaxell]

I know it's a bit late, but one thing that jumped out to me is using Expr as a base for the attribute. Expressions have a return value. It also isn't a statement since it can't appear in a control-flow. I would do something similar to trait-use in this context, so adding a new Op type for the attribute.

[eric-therond]

no problem for the delay, better late than never ;) thank you for your suggestion. I did changes to the PR. Let me know if it's what's you had in mind.

[ircmaxell]

Looks great, thanks for the contribution!