Makes Timer#stop a no-op unless it has first been started.

On running rspec3 with no examples, the elapsed timer would receive stop, then start, resulting in an error on trying to calculate @stopped_at - @started_at : 

```
/Users/jon/.rvm/gems/ruby-2.1.0/gems/activesupport-4.0.2/lib/active_support/core_ext/time/calculations.rb:263:in `-': can't convert nil into an exact number (TypeError)
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/activesupport-4.0.2/lib/active_support/core_ext/time/calculations.rb:263:in `minus_with_duration'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/activesupport-4.0.2/lib/active_support/core_ext/time/calculations.rb:274:in `minus_with_coercion'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/ruby-progressbar-1.4.0/lib/ruby-progressbar/components/timer.rb:9:in `start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/ruby-progressbar-1.4.0/lib/ruby-progressbar/base.rb:162:in `with_timers'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/ruby-progressbar-1.4.0/lib/ruby-progressbar/base.rb:30:in `block in start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/ruby-progressbar-1.4.0/lib/ruby-progressbar/base.rb:173:in `with_update'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/ruby-progressbar-1.4.0/lib/ruby-progressbar/base.rb:28:in `start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/fuubar-1.3.2/lib/fuubar.rb:30:in `block in start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/fuubar-1.3.2/lib/fuubar.rb:81:in `with_current_color'
        from /Users/jon/.rvm/gems/ruby-2.1.0/gems/fuubar-1.3.2/lib/fuubar.rb:30:in `start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/reporter.rb:120:in `block in notify'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/reporter.rb:119:in `each'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/reporter.rb:119:in `notify'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/reporter.rb:61:in `start'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/reporter.rb:51:in `report'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/command_line.rb:25:in `run'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/runner.rb:103:in `run'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/lib/rspec/core/runner.rb:33:in `invoke'
        from /Users/jon/.rvm/gems/ruby-2.1.0/bundler/gems/rspec-core-a173f6314c77/exe/rspec:4:in `<top (required)>'
        from bin/rspec:16:in `load'
        from bin/rspec:16:in `<main>'
```