Namespace support

[ajiyoshi-vg]

• sometimes it is need that KVS supports a namespace feature to guarantee the uniqueness of keys.
• I implemented namespace feature using bucket of boltdb
• I do not provide other storage implementations
• fix some struct receiver problem

[yowcow]

Nice implementation to manage namespaced backend, and I really appreciate it.

One thing, I'd like to have these changes tested!

Thanks!!!1

[yowcow]

Thanks!