Updates in light of peer review

• Re-implemented functions that access ClinicalTrials.gov or DRKS.de
  with the polite R package to ensure that the sites' rules
  regarding scraping are observed, and to limit the number of requests
• Fix bug where non-English locale prevents correct parsing of month
  names
• Improvements to error-catching during download
• New columns indicating the precision of dates in the original data
  source
• All functions now check that input is well-formed before executing
  and provide informative errors otherwise
• clinicaltrials_gov_download() and drks_de_download() now return
  a data frame of results if output_filename is not specified
• clinicaltrials_gov_version() and drks_de_version() now return
  named lists rather than vectors
• Added default version numbers to clinicaltrials_gov_version() and
  drks_de_version()
• More extensive unit testing
• Fix bug in retrieving "gender based" in clinicaltrials_gov_version()
• Add "quiet" downloading option
• Improved documentation