Faster countna

Fair warning: I've never done a pull requests before and I'm still fuzzy on the apropriate GitHub + julia package workflow so I may have mucked up something.

I found myself repatedly doing `countna` on big DataArrays so I went looking for performance improvement opportunities. `countna` in this pull request is up to 380x faster for DataArrays for large arrays and up to 3x faster for PooledDataArrays.

I've also included some tests for `countna`.