Merge branch 'release/0.6.0' into develop
write json-like data for Datum.new

permit symbol hash keys in Jubatus::Common::Datum.new
Merge pull request #23 from katsyoshi/datum_initializer_argument_hash_permit_symbol_keys

write json-like data for Jubatus::Common::Datum.new
bump version
Merge branch 'release/0.6.1'