ENH: automatic rpy2 instance conversion

Derived from #7309. Create a wrapper for `robjects.r` in `pandas.rpy.common` to perform automatic pandas `DataFrame` and `Series` conversion. `Series` will be converted to R data.frame to preserve rownames (index).

If looks OK, I'll modify the doc (#7309) based on following API.

```
import pandas as pd
import pandas.rpy.common as com

iris = com.load_data('iris')
com.r.assign('iris', iris)
returned = com.r['iris']
type(returned)
# <class 'pandas.core.frame.DataFrame'>

df = pd.DataFrame(np.random.randn(20, 5),
                  index=pd.date_range(start='2011/01/01', freq='D', periods=20))
com.r.assign('df', df)
returned = com.r['df']
type(returned)
# <class 'pandas.core.frame.DataFrame'>

s = pd.Series(np.random.randn(20), name='test')
com.r.assign('s', s)
returned = com.r['s']
type(returned)
# <class 'pandas.core.frame.DataFrame'>
```
It might be better to use the interface provided, i.e., instead of `robj.r.__getattribute__(attr)`, just do `getattr(robj.r, attr)`. Same for the below methods: just call their respective toplevel functions or behavior as you would if you were a user. Sometimes Python itself performs ops on the result of a special method call, e.g., for rich comparisons Python will automatically compare the `id`s of two objects if either of their comparison methods of the same name return `NotImplemented`. This is done internally in Python, but if you directly call the method like `__eq__` you don't get this convenience.