rvertnet v0.6.0

Added Code of Conduct.

NEW FEATURES

• Now using crul package for HTTP requests instead of httr (#57)
• Note that verbose parameter has been replaced with messages throughout
  the package.
• Now with function for search for trait data: traitsearch() (#55)

DEFUNCT AND DEPRECATED

• All dump functions are now defunct. Those functions tried to help users
  work with bulk Vertnet data - the setup has gotten too complex (#56)

MINOR IMPROVEMENTS

• Improvements to documentation for traitsearch() function on what
  fields have given data (#58) thanks @gaurav
• vertsearch() and searchbyterm() gain new parameter only_dwc, which
  allows to optionally only return Darwin Core fields

BUG FIXES

• Small fix to vertsummary() (59)