ENH: allowed read_csv to accept a basestring as the buffer and interpret as StringIO

[jreback]

already do this in read_json

first check if its a valid filename (which is safe in python)
if so, then open and process
else try to use as a StringIO(buffer)

[jreback]

the downside of this is that a non-existant file returns an empty frame which is odd

``
(Pdb) path
'1yLtjInbe1.csv'
(Pdb) self.read_csv(path)
Empty DataFrame
Columns: [1yLtjInbe1.csv]
Index: []