Add 'logging' gem.

Also needed to bump simplecov to a version compatible with the newer version
of multi_json.
Use loggers from the `logging` gem.

Instead of having to pass around Logger instances, we can just give
each class a logger, so we can get customisable log settings across
the app without having to pass and store log objects.
Pull logging into index classes from the Rakefile.
Merge branch 'logging'

Conflicts:
	lib/elasticsearch/index.rb - conflict with removing 'section'