- uniform number->date cell conversion
- update SSF to 0.9.0 (for is_date)
- namespaced rich str xml
- license file changed to appease GH automatic license detector

|     id      | author       | comment                                 |
|:------------|:-------------|:----------------------------------------|
| closes #581 | @Aymkdn      | XLSX cellDates + SSF.is_date            |
| closes #126 | @elad        | same as #581 h/t @SystemParadox         |
| closes #59  | @vratiu      | same as #581                            |
| closes #279 | @varunoberoi | raw json output uses date               |