Rename CancellablePolling to Cancellable

Polling is just one example where cancelling can be applied
Rework the examples structure

Every example is now executable and more descriptive about itself
Support for canceling action from web console
Minor web console enhancements
Restructurize the README
Simplify detection of cancellable action
Fix usage of MiniTest::Test vs. MiniTest::Spec

Otherwise, when running `test/persistance_adapters_test.rb` one gets
strange errors such as `undefined method `assert_raises' for nil:NilClass`