reduceby defaults to item in no ``init`` case
get([0], ...) return tuple
pluck([0], ...) returns tuple
Merge pull request #184 from mrocklin/reduceby-init

reduceby defaults to item in no ``init`` case
Merge pull request #190 from mrocklin/get-list-len-1

`get` and `pluck` on lists of length 1
rename groupby func argument to key
countby supports non-callable key function
Merge branch 'master' into support-non-callables
use non-callable arg to simplify reduceby example