read_csv should default to index_col = 0
#### Code Sample

```python
df.to_csv(file_path)
df = pd.read_csv(file_path)
```
#### Problem description

Currently, the default CSV writing behaviour is to write the index column.  The default reading behaviour, however, is to assume there is no index column in the file. This is not intuitive when writing and reading files.

The expected behaviour is that a file which is written without any index option it should be able to be read without any index option. However, the default writing and reading behaviour results in an unnamed column.

One fix is to change the default index_col for read_csv, another option is to change the default index boolean for to_csv. The former is probably preferable as it preserves information.