Get rid of warnings from rspec-core's library and spec code.

This pull request makes rspec-core's own code warning free when running its spec suite. The only remaining warnings are from its dependencies:

```
pete@balloon:~/projects/rspec-core$ rake spec
/home/pete/.rvm/rubies/ruby-1.9.3-p392/bin/ruby -w -S rspec ./spec/autotest/discover_spec.rb ./spec/autotest/failed_results_re_spec.rb ./spec/autotest/rspec_spec.rb ./spec/command_line/order_spec.rb ./spec/rspec/core/backtrace_cleaner_spec.rb ./spec/rspec/core/command_line_spec.rb ./spec/rspec/core/configuration_options_spec.rb ./spec/rspec/core/configuration_spec.rb ./spec/rspec/core/deprecations_spec.rb ./spec/rspec/core/drb_command_line_spec.rb ./spec/rspec/core/drb_options_spec.rb ./spec/rspec/core/dsl_spec.rb ./spec/rspec/core/example_group_spec.rb ./spec/rspec/core/example_spec.rb ./spec/rspec/core/filter_manager_spec.rb ./spec/rspec/core/formatters/base_formatter_spec.rb ./spec/rspec/core/formatters/base_text_formatter_spec.rb ./spec/rspec/core/formatters/documentation_formatter_spec.rb ./spec/rspec/core/formatters/helpers_spec.rb ./spec/rspec/core/formatters/html_formatter_spec.rb ./spec/rspec/core/formatters/json_formatter_spec.rb ./spec/rspec/core/formatters/progress_formatter_spec.rb ./spec/rspec/core/formatters/snippet_extractor_spec.rb ./spec/rspec/core/formatters/text_mate_formatter_spec.rb ./spec/rspec/core/hooks_filtering_spec.rb ./spec/rspec/core/hooks_spec.rb ./spec/rspec/core/kernel_extensions_spec.rb ./spec/rspec/core/memoized_helpers_spec.rb ./spec/rspec/core/metadata_spec.rb ./spec/rspec/core/option_parser_spec.rb ./spec/rspec/core/pending_example_spec.rb ./spec/rspec/core/project_initializer_spec.rb ./spec/rspec/core/rake_task_spec.rb ./spec/rspec/core/reporter_spec.rb ./spec/rspec/core/resources/a_spec.rb ./spec/rspec/core/rspec_matchers_spec.rb ./spec/rspec/core/ruby_project_spec.rb ./spec/rspec/core/runner_spec.rb ./spec/rspec/core/shared_context_spec.rb ./spec/rspec/core/shared_example_group_spec.rb ./spec/rspec/core/world_spec.rb ./spec/rspec/core_spec.rb
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/childprocess-0.3.9/lib/childprocess/unix/process.rb:46: warning: assigned but unused variable - pid
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/lang/ruby.rb:55: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
Run options:
  include {:focus=>true}
  exclude {:ruby=>#<Proc:./spec/spec_helper.rb:118>}

All examples were filtered out; ignoring {:focus=>true}
.........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................../home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
./home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
./home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
./home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
/home/pete/.rvm/gems/ruby-1.9.3-p392@rspec-core/gems/syntax-1.0.0/lib/syntax/common.rb:99: warning: character class has duplicated range: /\?(\\[^\n\r]|[^\\\n\r\s])/
.......................................................................................*.....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Pending:
  ::DRbCommandLine with server running outputs colorized text when running with --colour option
    # figure out a way to tell the output to say it's tty
    # ./spec/rspec/core/drb_command_line_spec.rb:95

Finished in 1.91 seconds
1163 examples, 0 failures, 1 pending

Randomized with seed 34559

```

The most questionable change is the one in `spec/rspec/core/example_spec.rb`, since I wasn't able to figure out how to make those warnings go away in a cleaner way.

Please let me know what you think, and thanks for making the specs so easy to run and understand!