allow overloading of a.b field access syntax

[StefanKarpinski]

Brought up here: #1263.

[stevengj]

The ability to use dots as syntactic sugar for mutator/accessor methods would be nice for lots of things. I've always appreciated this in languages that provide it, so that you can turn structure fields into more complicated abstractions without breaking the API.

[toivoh]

+1

[JeffBezanson]

I have an absolutely awesome way to implement this.

[johnmyleswhite]

Interested in talking about it? I know that Tom Short is really interested in having this for DataFrames, although I've come to be increasingly skeptical about the wisdom of using this feature.

[stevengj]

This would make calling Python code (via PyCall) significantly nicer, since currently I'm forced to do a[:b] instead of a.b.

[stevengj]

@JeffBezanson, any chance of having this for 0.3? Would be great for inter-language interop, both for PyCall and for JavaCall (cc @aviks).