DOC/ENH: read_csv dialect parameters should accept string
According to the doc of read_sql, dialect  can be string or a csv.Dialect instance.
However this is not the case. If you look at the code, it can only really take a csv.Dialect instance. i.e. this doesn't work:

``` python
fh = io.StringIO(u"pomme\tpoire\n1\t\2")
pd.read_csv(fh, dialect='excel-tab')
```

but, this works:

``` python
fh = io.StringIO(u"pomme\tpoire\n1\t\2")
pd.read_csv(fh, dialect=csv.get_dialect('excel-tab'))
```

I would expect it to be able to take any string returned by the csv.list_dialects() function, similar to the way csv.reader works. So we can either remove the mention of string from the doc, or make it work as intended by checking if the dialect string is part of csv.list_dialects(). Let me know if this makes sense and I'll submit a PR.
