Add support for for Ruby 2.2 GC.stat key names.

• Compatibility with Rails 4.0.
• Enable Travis-ci.
• Add controller#action in the gcstat_logfile store.

• Add missing files to gemspec.

• Add configuration file: config/memory_tracker.yml.
• Allow loading the stores by name after configuration settings.
• Pass store configuration into intialize config hash.
• Add dependency to activesupport for mattr_accessor and class_attribute.