Adds Travis CI integration

Also adds a few changes that were causing Travis CI failures and
confusing/unnecessary warnings.
Makes the subclass implementations explicit so developers can see API

The current implementation assumes an API which for new developers is
rather difficult.  Doing this lets anyone wanting to add a new
fulfillment service explicitly know what the Service interface is
supposed to be.
Merge pull request #21 from csaunders/feature/adds_explicit_api_to_service_superclass

Makes the subclass implementations explicit so developers can see API
Dropping 1.8.7 support, lets get testing against 2.0.0
Change activesupport dependency to 3.2.9
Fix deprecation warning from mocha

Deprecation warning:

> *** Mocha deprecation warning: Change `require 'mocha'` to `require
> 'mocha/setup'`.
Merge pull request #23 from Shopify/ruby-2.0.0

Ruby 2.0.0 support
Update travis image on README

add link to travis CI image.
Updates list of project contributors
Add some contribution guidelines to the readme