Add `ResultSet#column_names`. Fixes #11

spec/result_set_spec.cr

@@ -56,4 +56,12 @@ describe DB::ResultSet do
 
     cols.should eq(["c0", "c1"])
   end
+
+  it "gets all column names" do
+    with_dummy do |db|
+      db.query "1,2" do |rs|
+        rs.column_names.should eq(%w(c0 c1))
+      end
+    end
+  end
 end

src/db/result_set.cr

@@ -61,6 +61,11 @@ module DB
     # Returns the name of the column in `index` 0-based position.
     abstract def column_name(index : Int32) : String
 
+    # Returns the name of the columns.
+    def column_names
+      Array(String).new(column_count) { |i| column_name(i) }
+    end
+
     # Reads the next column value
     abstract def read