Riak 1.4 Feature: Client-specified timeouts

Riak 1.4 introduces client-specified timeouts for get, put, delete,
list-keys, and list-bucket operations. A `timeout` option,
specified in milliseconds, should be added to the relevant client
methods.

- [x] Add timeout option to `get_object`/`fetch_object`
- [x] Add timeout option to `store_object`
- [x] Add timeout option to `delete_object`
- [x] Add timeout option to `list_keys`
- [x] Add timeout option to `buckets`