[MRG] enable stack traces for any logging level

```python
from rootpy import SHOW_TRACE
from rootpy.vector import LorentzVector
import logging

SHOW_TRACE.enabled = True
SHOW_TRACE.level = logging.WARNING

def func():
    vect = LorentzVector(0, 0, 0, 0)
    vect.Eta()

func()
```

```
  File "test.py", line 12, in <module>
    func()
  File "test.py", line 10, in func
    vect.Eta()
WARNING:ROOT.TVector3.PseudoRapidity] transvers momentum = 0! return +/- 10e10
```