custom formatters for to_csv
- [ ] date formatting #2583, PR #4313, #6797
- [ ] string spacing (justification?) #4158
- [ ] float format #2502, #2069
- [ ] int format, #6502
- [ ] timedelta fromat #6783 

[SO question](http://stackoverflow.com/questions/18425400/printing-to-csv-file-with-automatic-width)

something like

``` python
df.to_csv(format='%10.4f', sep=' ')
```
