define WriteableStore interface, divorced from ReadableStores. A Store is now just the composition of a ReadableStore and WritableStore which allows the types to tell the full story. This should not break existing interfaces
writable stores should should themselves be Closable
update readme with new WritableStore ref
extra put semantics should not depend on reading semantics
contravariant value types for WriteableStores, don't force the Option type for values
Merge pull request #180 from softprops/writeable-stores

Writeable stores