When reading CSV, float-like strings lacking exponent digits are accepted as floats

The data-frame

``` python
DataFrame({'x': [2.5], 'y': [42], 'z': ['2E']})
```

does not round-trip correctly via CSV.  The string `'2E'` is interpreted as a valid float, but it should not be (according to man strtod(3), which seems a reasonable spec).
