Changes in version 0.4.4

- Change API version from 21 to 22.
- Modify the vignette "rdbnomics-tutorial.Rmd" to be backward compatible with
  R Markdown v1 (i.e. without pandoc or rmarkdown).
- Add appendix to the vignette "rdbnomics-tutorial.Rmd".
- New function "rdb_providers" to return the available providers.
- New function "rdb_last_updates" to show the last updates.
- Dates and timestamps are transformed using as.Date and as.POSIXct.

Changes in version 0.4.3

- First release on CRAN.
