[WIP] Feature/csv to sql natively

When finished, this PR should address #352. New `into` function calls out to to native COPY function of various DBs.  Has optional `if_exists` argument (similar to pandas) to handle three cases: replace, append, and fail

```python
into(sql,csv, if_exists="replace")
```

This PR currently handles the native copy calls for:

- PostgreSQL
- MySQL
- SQLITE


- [x] Travis CI PostgreSQL Testing
- [ ] Travis CI MySQL Testing

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Sorry to keep having small issues, but one more thing. 

There is no default fail case to this function.  What if none of these conditions are met?  What if platform == windows and dbtype == sqlite?  We should have a general catch-all that defaults to `sql.extend(csv)`

I suggest changing the many `if`s to `elif`s, including a `platform` check for windows as follows

```
import platform
platform.system()  # I think this is the right thing to check, not sure
```

and then having a final `else` that calls `sql.extend(csv)` 

I can do this if you prefer to be done with this PR.