Add column lookup with Base.getproperty?

[ararslan]

Once this package requires 0.7 at a minimum, we can define column lookup in terms of getproperty in addition to getindex, so that one can access a column x of a DataFrame df as df.x in addition to df[:x].

We could consider a full replacement, but continuing to allow df[:x] permits fetching multiple columns at a time via df[:, [:x, :y, :z]]. I'm not sure how that would translate to the getproperty syntax.

[nalimilan]

Agreed. It would make sense to couple this to a port to NamedTuple (#1335), in particular since filter would then pass a NamedTuple (which supports the dot syntax) to the user-provided predicate.