Added `complete_cases` for weeding out NA's in AbstractDataFrames.
Minor indexing changes (.< & .>).
In indexing, move the array index out to ref().
Another try at indexing with Vector{Range1}.
Rework | and & based on `union` and `intersect` of `Vector{Range1}`.
Minor change.
Allow column groups in formulas.
Small updates to motivation documentation
minor function reference tweaks
isna(AbstractArray) now returns false array of the same size

instead of a scalar false, which was weird
Incorporate code review suggestions for groups in formulas.
Merge pull request #46 from doobwa/formulas-w-groups

Allow column groups in formulas.
Enforce unique names at the Index level.
Add start of a benchmark test (issue #40).
Tweak to datavec benchmark case.
Fix ! for Indexer.
Update experimental_indexing.jl with faster lookups and logical evaluations.
A few more perf tests.
Add left, right, and outer joins (issue #17).
A couple of methods for model_frame and model_matrix.