ENH: .from_dataframe() convenience function

[jreback]

In [7]: import ibis

In [8]: df = pd.DataFrame({'A': [1, 2, 3]})

In [9]: t = ibis.pandas.connect({'df': df}).table('df')

In [10]: t.A+1
Out[10]: 
ref_0
PandasTable[table]
  name: df
  schema:
    A : int64

Add[int64*]
  left:
    A = Column[int64*] 'A' from table
      ref_0
  right:
    Literal[int8]
      1

would be nice to write this like

from ibis.pandas.api import from_dataframe
t = from_dataframe(pd.DataFrame(.....))

makes interop with pandas that much easier