Introduce rootpy.ROOT, a pluggable replacement for `import ROOT`

This module allows one to import classes with ROOT names from `rootpy.ROOT`
and obtain "rootpy-ized" classes.

In addition it overloads `gPad`. This is experimental. Note that the current implementation returns the gPad at the moment it is requested, so:

```python
from rootpy.ROOT import gPad
```

.. will not work as expected, it will only return the `Canvas` which is currently in use at the point of the import. We can decide if we want to mimick PyROOT's solution to this or not.

Note that in addition there is a `R` property on the module which returns normal ROOT in case it is needed.

I've put a bit of documentation into the module but it isn't yet cross-referenced from the rest of the documentation.