fix issue #343: `from rootpy import ROOT` is not the same as `import rootpy.ROOT as ROOT`
ROOT.py: support both ROOT.TX for asrootpy'd ROOT lookup and and ROOT.X for direct rootpy lookup
ROOT.py: asrootpy automatically after __init__ to preserve ROOT's __init__
ROOT.py: add unit test
ROOT.py: docs
mention ROOT.py in README.rst
Merge pull request #408 from ndawe/master

[MRG] rootpy.ROOT improvements