read_csv: na_values=dict and index_col don't play nice together
It looks like passing a dict for read_csv.na_values doesn't work if there's also an index_col:

```
In [1]: import pandas

In [3]: import StringIO

In [4]: pandas.read_csv(StringIO.StringIO("a,b\n0,NA\n"), na_values={}, index_col=0)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/bbloniarz/src/pandas-git/<ipython-input-4-f407e87fa8a9> in <module>()
----> 1 pandas.read_csv(StringIO.StringIO("a,b\n0,NA\n"), na_values={}, index_col=0)

/home/bbloniarz/src/pandas-git/pandas/io/parsers.py in read_csv(filepath_or_buffer, sep, dialect, header, index_col, names, skiprows, na_values, thousands, comment, parse_dates, keep_date_col, dayfirst, date_parser, nrows, iterator, chunksize, skip_footer, converters, verbose, delimiter, encoding, squeeze)
    234         kwds['delimiter'] = sep
    235 
--> 236     return _read(TextParser, filepath_or_buffer, kwds)
    237 
    238 @Appender(_read_table_doc)

/home/bbloniarz/src/pandas-git/pandas/io/parsers.py in _read(cls, filepath_or_buffer, kwds)
    189         return parser
    190 
--> 191     return parser.get_chunk()
    192 
    193 @Appender(_read_csv_doc)

/home/bbloniarz/src/pandas-git/pandas/io/parsers.py in get_chunk(self, rows)
    758         if not self._has_complex_date_col and self.index_col is not None:
    759             index = self._get_simple_index(zipped_content)
--> 760             index = self._agg_index(index)
    761         else:
    762             index = Index(np.arange(len(content)))

/home/bbloniarz/src/pandas-git/pandas/io/parsers.py in _agg_index(self, index, try_parse_dates)
    878             if try_parse_dates and self._should_parse_dates(self.index_col):
    879                 index = self._conv_date(index)
--> 880             index, na_count = _convert_types(index, self.na_values)
    881             index = Index(index, name=self.index_name)
    882             if self.verbose and na_count:

/home/bbloniarz/src/pandas-git/pandas/io/parsers.py in _convert_types(values, na_values)
   1065         result = lib.maybe_convert_numeric(values, na_values, False)
   1066     except Exception:
-> 1067         na_count = lib.sanitize_objects(values, na_values, False)
   1068         result = values
   1069 

/home/bbloniarz/src/pandas-git/pandas/lib.so in pandas.lib.sanitize_objects (pandas/src/tseries.c:96732)()

TypeError: Argument 'na_values' has incorrect type (expected set, got dict)
```
