Merge pull request #223 from Ramda/master

empty and of for fantasy-land
Merge pull request #224 from Ramda/master

chain; flatten revisited; "of"
Merge pull request #226 from Ramda/master

small flat correction; alias for flatten
Merge pull request #227 from Ramda/master

arity fixes for of and empty
add R.is
Merge pull request #228 from davidchambers/is

add R.is