[MRG] rootpy.ROOT improvements

Fixes #343

```python                                                                    
>>> import rootpy.ROOT as ROOT                                                  
>>> ROOT.TFile                                                                  
<class 'rootpy.TFile_asrootpy'>                                                 
>>> f = ROOT.TFile('test.root', 'recreate')                                     
>>> # the TFile is asrootpy'd after ROOT's init is called:                      
>>> f                                                                           
File('test.root')
>>> # similarly for TH1F
>>> h = ROOT.TH1F("name", "title", 10, 0, 1)
>>> h
Hist('name')
>>> h.TYPE
'F'                                                               
>>> # access rootpy subclasses in the same way                                  
>>> ROOT.File                                                                   
<class 'rootpy.io.file.File'>                                                   
>>> # underlying ROOT can be accessed through the R property.                   
>>> ROOT.R.TFile                                                                
<class 'ROOT.TFile'>                                                            
>>> # also import specific names                                                
>>> from rootpy.ROOT import TFile                                               
>>> TFile                                                                       
<class 'rootpy.TFile_asrootpy'>
```