Clean up rake tasks

* Remove metric_fu tasks
  - metric_fu now has its own runner
  - metric_fu is not part of the bundle, so these tasks were not
    available when run with `bundle exec`
  - loading these tasks takes an insanely long time
* Delay loading yaml dependency until needed
* Delay requiring Reek sniffer until needed
* Remove dependency declarations for obsolete tasks
* Remove unused Rakefile constants
* Remove needless dependencies
* Do not manipulate the load path: The 'lib' directory is already added
  to the load path by 'bundler/gem_tasks'.
* Remove needless square brackets
* Expect config/ to exist: its contents is in git, and it should not be
  removed by `rake clobber`.
Make rake test:quality work

The pattern option does not seem to work if it does not start with
'spec'.
Merge pull request #296 from troessner/fix-test-quality-task

Make rake test:quality work
Merge pull request #293 from troessner/clean-up-rake

Clean up cruft from rake tasks
Require Ruby 1.9.2 or up
Merge pull request #298 from troessner/issue-require-ruby-1-9-2

Require Ruby 1.9.2 or up
Remove debugger gems from the bundle

Removes debugger, byebug and pry_debug from the bundle, and fixes
instructions in README.md accordingly.
Merge pull request #299 from troessner/issue-clean-up-debugger-gems

Clean up debugger gems
Add support for detecting specific smells

This adds the command-line option --smell to specify a smell to detect.
This option can be applied multiple times.

Some refactoring was needed to be able to pass the smells to detect down
to where the actual detection takes place.