Support project specific vendor/bundle install option

Bundler allows you to install required gems to a folder inside the project instead of the default gem install location. I added the require statements to slogger.rb (as per instructions on http://bundler.io) to load the bundled environment.

This should enable you to use 'bundle install --path vendor/bundle' (instead of 'bundle install') to install all gems required by slogger to a vendor/bundle folder inside the Slogger folder.