Unintuitive tax query 'operator' values

[scribu]

Currently, the accepted values for the tax_query 'operator' arg are:

AND, IN and NOT IN.

It would be better if they were instead:

AND, OR and NOT.

which would be more intuitive and consistent with wp_list_filter().

Obviously, we would still have to keep backwards compatibility with the old values.

Agreed, OR rather than IN makes more sense and is more consistant. Same with NOT.