raw redis interface via cachedb

it would be very productive to have redis as cache backend. 

I have added cachedb_raw_redis with support to run native redis commands like sadd spop get set etc.

Eg:
```
cache_raw_redis("redis:group1","SADD","abcd","9191","10");
cache_raw_redis("redis:group1","SPOP","abcd",$var(spop));
```

However this does not support avp variables. Hence commands with multiple values in reply are not supported yet.

I would love to add that support with some help.