auk 0.2.0

• New function, auk_split(), splits EBD up into multiple files by species
• New object, auk_sampling, and associated methods for working with the sampling data only
• New function, auk_select(), for selecting a subset of columns
• auk_date() now allows filtering date ranges across years using wildcards, e.g. date = c("*-05-01", "*-06-30") for observations from May and June of any year
• New function, auk_state() for filtering by state
• Now using AWK arrays to speed up country and species filtering; ~20% speed up when filtering on many species/countries
• Allow selection of a subset of columns when filtering
• Remove free text columns in auk_clean() to decrease file size
• Updated to work with Feb 2018 version of EBD
• Fixed broken dependency on countrycode package