Unified logger that writes to a log file.
Could eventually also write to a redis queue, mongo collection, and hoptoad.

Unilogger is composed at runtime of a Logger instance which delegates log messages to a set of emitters.
The emitters do the actual work of sending log messages to stderr, a file, redis, or elsewhere.

Unilogger will initialize itself from a logger.yml or logger.yml.erb configuration file, if present.

The configuration file should contain an entry named after the current environment, e.g.
"development", "test", "production", etc.

Logger options
--------------
level:      overall level; messages of severity less than this will not be emitted regardless of the level of the emitter.
emitters:   a list of emitters.

Standard (log file) emitter
---------------------------
kind:       LogFileEmitter
logdev:     "stderr", "stdout", or a filename
shift_age:  "daily", "weekly" or "monthly" to rotate based on the calendar
            if rotating based on file size, number of files to keep
            default 7
shift_size: a file size, to rotate based on the file size
            default 1048576

Redis emitter
-------------
kind:       RedisEmitter


Example logger.yml
------------------
stderr:         &stderr
  logger:
    logdev:     stderr

standard:       &standard
  logger:
    logdev:     "<%= ENV["RACK_ENV"] || "development" %>.log"
    shift_age:  daily

development:
  level:        0
  emitters:     [ *standard ]

test:
  level:        1
  emitters:     [ *stderr ]