Processes can now use a new LIVERESOURCE_DB environment variable to
specify which Redis database to use for all LiveResource::RedisClients.

The tests now use Redis DB 15.  Tests no longer assume they own all
Redis databases.  That is, "Redis.new.flushall" is no longer used.
Likewise, "Redis.new" is no longer used.  This allows one, for example,
to run the LiveResource tests on a system where other processes are also
using Redis (e.g. a system that is using LiveResource for
production/development code).

lib/live_resource/redis_client.rb
  Use either LIVERESOURCE_DB or the DEFAULT_REDIS_DB when creating the
prototype redis instance.

test/test_helper.rb:
  Expand Test::Unit::TestCase to include 2 new helper methods,
flush_redis and redis_dbsize.  The former does a flushdb instead of
flushall.

test/...:
  Tests no longer directly access Redis via "Redis.new".  They either
use the new Test::Unit::TestCase helpers or get a redis instance via
LiveResource::RedisClient.redis to ensure it is pointing to the correct
database.