implement interpreter and demo + bug fixes

- implement interpreter and demo
- replace StandardError with appropriate subclasses
- implement division
- rewrite Polynomial.simplify()

lib/ruby_algebra/command.rb

@@ -3,6 +3,11 @@
 module RubyAlgebra
   # Команда интерпретатора
   class Command
+    attr_reader :type
+
+    def type
+      raise NotImplementedError
+    end
   end
 
   # Команда присвоения
@@ -39,20 +44,29 @@ module RubyAlgebra
       end
       result
     end
+
+    def type
+      :assignment
+    end
   end
 
   # Команда вывода на экран
   class DisplayCommand < Command
-    attr_reader :polynomial
+    attr_reader :item
 
-    def initialize(polynomial)
-      @polynomial = polynomial
+    def initialize(item)
+      @type = :display
+      @item = item
     end
 
     def to_s
       result = "Display command: "
-      result += @polynomial.to_s
+      result += @item.to_s
       result
     end
+
+    def type
+      :display
+    end
   end
 end