Feature flag for the volunteer opportunities feature

We have rewritten this with 2 feature flags :volunteer_ops_create and :volunteer_ops_list.

We created ActiveRecord feature flags inspired by the rollout gem (which needs Redis.)

The flag works by selecting different code paths, based on the value of `Feature.active?(:flag_sym)`. The two flags supported are `:volunteer_ops_create` and `:volunteer_ops_list`.

Run 'bundle exec rake db:migrate` to add the Feature table.

A feature is inactive until you create it _and_ activate it, which you can do from the rails console.

To create a flag

    Feature.create(:name => :volunteer_ops_create)

To activate 

    Feature.activate(:volunteer_ops_create)

and to deactivate it

    Feature.deactivate(:volunteer_ops_create)


We disabled the links in the navbar, the 'create a volunteer opportunity' button in the organization show page, 
and all the routes associated with volunteer opportunities. 
 (We reverted commit 114659985b230d0d45fae27d2db0c0847094e90c , and so restored the commented out code path and scenario that provides the navbar links.)
 
Rdoc style docs are in app/models/feature.rb
Cucumber features are added to features/volunteer_opportunities/
I propose a refactoring to remove the #feature_active? view helper, so that we send messages to the Feature class directly from the view.  I haven't yet pushed that to this pull request branch, pending discussion. See https://github.com/dcorking/LocalSupport/commit/6b36a846fbc57b06ab3564112716db6f6c2b9d36 to review the diff of my proposal