Fix duplicate loading of method_decorators/version

The lib/method_decorators/version.rb file was being loaded twice when
running RSpec with bundler because method_decorators.gemspec and
lib/method_decorators.rb required it in 2 different ways.
Change require statement in decorators.rb

This helps prevent loading files twice. Instead of requiring the full
path to the file, it requires
'method_decorators/decorators/name_of_decorator'.
Merge pull request #7 from tobyhs/load_version_once

Fix duplicate loading of method_decorators/version
Merge pull request #8 from tobyhs/fix_require_decorators

Change require statement in decorators.rb
Move decorators under MethodDecorators:: scope