Upgrade to 0.5.8

Features
* pass custom values as NA values
  * parameter change: `na_values` accepted by `datasets/create`

Error fixes
* fixing docs and typos in the docs
* recognize dates in JSON files
* cast win_type parameter correctly
* encapsulate query arguments (for `datasets`, `dframes`, and `observations`) in an object
* cast row endpoint params correctly
* counts update correctly on row `delete`
* allow deletion of calculations that failed (or already had their rows dropped)
* fix error when adding calculation after a calculation has failed