rootwait: failover on raw_input if no PyGUIThread

This is a solution to #226 

```
    if _processRootEvents is None:                                              
        log.warning(                                                            
            "unable to access ROOT's GUI thread either because "                
            "PyROOT's finalSetup() was called while in batch mode "             
            "or because PyROOT is using the new PyOS_InputHook "                
            "based mechanism that is not yet supported in rootpy "              
            "(PyConfig.StartGuiThread == 'inputhook' or "                       
            "gSystem.InheritsFrom('TMacOSXSystem')). wait() etc. will "         
            "instead call raw_input() and wait for [Enter]")
```

trigger with

```
from rootpy.plotting import Hist
from rootpy.interactive import wait
import ROOT

ROOT.gROOT.SetBatch(True)
# trigger PyROOT's finalSetup()
ROOT.kTRUE
ROOT.gROOT.SetBatch(False)

h = Hist(50, 0, 100)
h.Draw('hist')

wait()
```