Property declaration support in interfaces

[ghost]

In an effort to continue the work in this PR https://github.com/dropbox/djinni/pull/287, I've attempted to address the review comments and ensure the concerns have been addressed. Please review that original PR for more details.

Wherever possible I attempted to inject property-specific accessor information into the methods AST to share code generation. I was unable to do this at the parser level, since different languages have different needs.

[xianwen]

Hi, @MikeNachbaur: I noticed you haven't signed the CLA yet, could you please sign it here: https://opensource.dropbox.com/cla/ Thanks a lot!