Segment environment-checking into #load_configuration

Allow for easier subclassing / overriding of behavior
Break out version string for easier subclassing
Make #should_start public so librato-rails railtie can use it
Support a pre-configured tracker if one is present
Separate process locking from #should_start? check
Make log prefix configurable
Merge pull request #31 from librato/feature/support_librato_rails

Feature/support librato rails