CRAN release version 0.2.1

Minor improvements:

• Improved response and request handling. httr::GET is replaced by httr::RETRY, for automatic retries on failing API requests. On some occassions API requests were failing with the error message "Error : (2002) Connection refused", usually when the server was too busy - these requests now fail gracefully with an informative error message.
• Added a 1-second sleep function between each API call to reduce stress on server.
• Added functionality for checking internet connection prior to attempting API calls.
• Fixed issue where some iterative queries were failing due to a mismatch between the number of records the bioRxiv API estimated versus the number of actual records.