Count is number of non null for various backends

Backends:

- [x] `pandas` (already behaves this way, `compute_up` calls `NDFrame.count()`
- [x] `mongodb` (this PR)
- [x] `python` #728 
- [x] `numpy` #728 
- [ ] `bcolz`
- [ ] `spark`
- [x] `sql` (I believe this calls `COUNT(x)` which already behaves this way
- [ ] `pytables`