Rails 3 support \o/

This is heavily inspired from the excellent lograge gem by
Mathias Bacon Meyer.
Add rake to Gemfile to aid testing
Add hard dependency to activesupport, don't require actionpack anymore
Move requires closer to actual depencencies
The silence feature is optional for loggers
More tested environments
Return a sensible value when calling do_not_log! without a buffer
First tests of BufferedLogger
Add more buffer tests
Don't explicitly require minitest/spec anymore
Require forwardable for jruby
Exclude Ruby 1.8 builds on Rails 4
Additional test requires to please jruby and ruby 1.8
Restore compatibility to Ruby 1.8
Ensure that the logged message is a String
Add tests for 7b508ed08
Parse and evaluate JSON logs in tests
Extract message normalization
Refactor: simplify delegation of logger methods
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