Abstract _.pick and _.omit via pick wrapper

[megawac]

Use a function generator to create _.pick and _.omit as they're essentially the same function.

We can expect a major performance improvement from this changeset for all cases of _.omit with some perf hit on the properties as argument case for _.pick

[megawac]

Let me know if the old way is preferred

Old way:

keys = _.map(concat.apply([], slice.call(arguments, 1)), String);
iterator = function(value, key) { return !_.contains(keys, key); };

[megawac]

http://jsperf.com/invert-vs-map which should be used for this case?

[jashkenas]

We can expect a major performance improvement from this changeset for all cases of _.omit with some perf hit on the properties as argument case for _.pick

Feel like showing it in a little JSPerf?

[megawac]

I updated the pr from the original commit to be faster for the arguments case (slightly less terse now).

http://jsperf.com/underscore-1639/3

[jdalton]

This looks like a mixed bag across browsers. I don't see a clear win for this change.

[braddunbar]

...not to mention the added complexity. I rather prefer them separate.