Graceful Speedy Data

Features

• Improved graceful errors for fetch_ucirepo() and list_available_datasets()
  when resources are not found/available. (#3,
  thanks Prof. Ripley!)
• Speed up fetch_ucirepo() for large data frames by switching to using base
  functionals instead of growing a vector in a loop while sorting variable roles.
  (#6)

Bug fixes

• Fixed internal subset issue with fetch_ucirepo() when metadata from the
  repository had whitespace characters in the variable names. (#2)