Add DUMP

[hltbra]

Fixes #25

This pull request doesn't treat specially optimized cases, such as intsets and ziplists, but it's still a fully compatible output:

# dredis
$ redis-cli -p 6377
127.0.0.1:6377> set dredis_foo bar
OK
127.0.0.1:6377> dump dredis_foo
"\x00\x03bar\a\x00\x19\x8c\x9c\x93\xe6\xac(\r"
127.0.0.1:6377> zadd dredis_z 0 value0 1 value1
(integer) 2
127.0.0.1:6377> dump dredis_z
"\x03\x02\x06value0\x010\x06value1\x011\a\x00,\xf1\x99\xda#.3="

# redis
$ redis-cli -p 6379
127.0.0.1:6379> restore redis_foo 0 "\x00\x03bar\a\x00\x19\x8c\x9c\x93\xe6\xac(\r"
OK
127.0.0.1:6379> get redis_foo
"bar"
127.0.0.1:6379> restore redis_z 0 "\x03\x02\x06value0\x010\x06value1\x011\a\x00,\xf1\x99\xda#.3="
OK
127.0.0.1:6379> zrange redis_z 0 -1 withscores
1) "value0"
2) "0"
3) "value1"
4) "1"