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
add MergeableMemcacheStore
add comments, make class concrete
add unit test
contravariant value types for WriteableStores, don't force the Option type for values
change put to add, make retries configurable
Merge pull request #182 from rubanm/feature/memcache_mergeable

MergeableMemcacheStore
Merge pull request #180 from softprops/writeable-stores

Writeable stores
Add the Mergeable type
Add readme notes for memcached tests
Merge pull request #184 from rubanm/memcache_readme

Add readme notes for memcached tests
Merge pull request #183 from twitter/mergeable

Add the Mergeable type
Merge pull request #13 from twitter/develop

Merge from twitter/storehaus