auk 0.3.0

• Updated to 2018 taxonomy; new function get_ebird_taxonomy() to get taxonomy via the eBird API
• Better handling of taxonomy versions, many functions now take a taxonomy_version argument and use the eBird API to get the taxonomy
• auk_getpath() renamed auk_get_awk_path(), and added auk_set_awk_path()
• Added auk_set_ebd_path() and auk_get_ebd_path() to set and get the
  EBD_PATH environment variable. Now users only need to set this once and just
  refer to the file name, rather than specifying the full path every time.
• Functions to prepare data for occupancy modeling: filter_repeat_visits() and format_unmarked_occu()
• New auk_bcr() function to extract data from BCRs
• Added bcr_codes data frame to look up BCR names and codes
• "Area" protocol added to auk_protocol() filter.
• auk_extent() renamed auk_bbox(); auk_extent() deprecated and redirects to auk_bbox()
• auk_zerofill() now checks for complete checklists and gives option to not rollup
• auk_rollup() now gives the option of keeping higher taxa via drop_higher argument
• auk_clean() deprecated
• Fixed package load error when EBD_PATH is invalid
• Fixed bug when reading files with a blank column using readr