Don't require order minimum for store credits by default

Further README tweak to show implementation of preference
Spree 2.0
removed spree_promo
another spree_promo that was commented out
Spree 2.1 compatibility.

Key changes:

1. Replace Rails 3 mass assignment with strong parameters
2. Update gemspec and Gemfile to reference unreleased 2.1.0.beta
3. Update JS and CSS files to refer to the broken out source files (spree_frontend, spree_backend) instead of spree_core
4. Remove unused model class
5. Reorganize engine class into a dedicated file, use include in lib/spree_store_credits.
6. Change translations to use Spree.t
7. Update FactoryGirl syntax
8. Add a number of specs
9. Update existing specs for compatibility with Spree 2.1 changes
10. Move spec/requests to spec/features
11. Add simplecov
12. Use database_cleaner for database cleanup
13. Update .travis.yml to run against all branches, remove notifications, and add xvfb
Typo Fix
Changing text to icon for user administration in backend
Merge pull request #46 from bonobos/icon

Changing text to icon for user administration in backend
Depend on any 2.1 Spree version
Use 2-1-stable branches for all gems
Remove debugger gem as a dependency
Use right version of spree_sample
Bump selenium-webdriver to 2.35.1
Add german translation.

Fixes #51
Use Spree::Price to parse the credit value.

Fixes #56
Change preference type from float to decimal

Fixes #57
Rename method load_order to load_order_with_lock

Fixes #59
Bump selenium-webdriver to 2.39.0
Use right factory name in store_credits_spec