Documentation fixes

[leeper]

• [x] destination argument in GenerateNotification has missing brackets
• [x] Update package description and remove mention of authenticate
• [x] Add "DoesNotExist" to AssignQualification's conditional.comparator description, add spacing between comparators, and put them in quotes
• [x] Remove documentation for authenticate
• [x] Change \url{} to \href{} in CreateHIT for response.group and hitlayoutid arguments
• [x] Change \url{} to \href{} in DisableHIT for response.group
• [x] Change \url{} to \href{} in DisposeHIT for response.group
• [x] Change \url{} to \href{} in GetAssignment for response.group
• [x] Change \url{} to \href{} in GetHIT for response.group
• [x] Change \url{} to \href{} in GetHITsForQualificationType for response.group
• [x] Change \url{} to \href{} in GetReviewableHITs for response.group
• [x] Change \url{} to \href{} in GetStatistic for response.group
• [x] Change \url{} to \href{} in SearchHITs for response.group
• [x] In GenerateExternalQuestion, note sandbox submit URL
• [x] Make valid HTMLQuestion structure in GenerateHITsFromTemplate example
• [x] Make GrantQualification example complete by creating qualification and polling for requests
• [x] Make RejectQualification example complete by creating qualification and polling for requests
• [x] Fix README for CRAN (check pandoc conversion)