Switch redis.c plugin from credis to hiredis.

Change the entire redis.c plugin to use libhiredis (tested with
libhiredis0.10) instead on credis. The libhiredis is supported in a number
of distributions like Debian or Ubuntu.

This patch keeps the same functionality that the old redis.c does.

Conflicts:
	src/redis.c
	src/types.db
use timeval. keep timeout in milliseconds for backwards compatibility.
Revert types for redis.c plugin.
remove all credis left. Migrate write_redis too.

Conflicts:
	README
Set the right order to parse the redis info.
Add memory_lua data type
Fix memory leak in redis.c
write_redis: re-add colon dropped in b7984797

When running f3706b0b87, the following command gets sent to redis:
  "ZADD" "collectd/hostname/entropy/entropy" "1415487432.000000" "1415487432:932"
Meaning the value actually stored, and later returned by redis is:
"<timstamp>:<value>".

b7984797 accidentally dropped the comma separating the timestamp and the
value, which leads the plugin to store a somewhat confusing value in
redis:
  "ZADD" "collectd/hostname/entropy/entropy" "1415487432.000000" "1415487432932"
redis: add missing constant
write_redis: fix format of commands sent to redis

The commands getting submitted to redis now look like this:
"ZADD" "collectd/hostname/entropy/entropy" "1415602051.335973024" "1415602051.335973024:823"
"SADD" "collectd/values" "hostname/entropy/entropy"

... which is the same as in the initial implementation, except for the
added decimals in the timestamp (the plugin was developped before
high-precision timestamps support was added to collectd).
write_redis: avoid passing a float/double to redisCommand()

... as it seems to not be well supported by hiredis 0.10.1 on Debian
7.0, leading to a segfault. Storing the string representation in a
variable instead is the compromise I found to make the plugin work on
this system.
Merge pull request #799 from mfournier/hiredis-switch

Switch redis & write_redis plugins from credis to hiredis