Implement basic condition DSL framework for tests:

Instead of interrupting task evaluation when condition is not met,
allow the whole task to be evaluated (including condition and
evaluation) so that the interpreter will get all tasks actions (whether
condition is met or not) and be able to query the condition.

* Modify Interpreter#process_task: test if task condition is met before
  applying the actions;
* Implement condition handling in Task and Task::DSL;
* Implement Condition and Condition::DSL (useless as they are, but
  needed to implement later test keywords as part of the condition DSL.

spec/producer/core/condition_spec.rb

@@ -0,0 +1,39 @@
+require 'spec_helper'
+
+module Producer::Core
+  describe Condition do
+    let(:expression)  { double('expression') }
+    let(:condition)   { Condition.new(expression) }
+
+    describe '.evaluate' do
+      let(:env)   { double('env') }
+      let(:block) { proc { :some_condition_code } }
+
+      it 'delegates to DSL.evaluate' do
+        expect(Condition::DSL)
+          .to receive(:evaluate).with(env) do |&b|
+            expect(b.call).to eq :some_condition_code
+          end
+        Condition.evaluate(env, &block)
+      end
+
+      it 'returns the evaluated condition' do
+        condition = double('condition')
+        allow(Condition::DSL).to receive(:evaluate) { condition }
+        expect(Condition.evaluate(env, &block)).to be condition
+      end
+    end
+
+    describe '#initialize' do
+      it 'assigns the expression' do
+        expect(condition.instance_eval { @expression }).to be expression
+      end
+    end
+
+    describe '#!' do
+      it 'returns the negated expression' do
+        expect(condition.!).to be !expression
+      end
+    end
+  end
+end