Ability to replace a node with another node or string

```
element = Oga::XML::Element.new(:name => 'div')
some_node.replace(element)
```

You can also pass a `String` to  `replace` and it will be replaced with
a `Oga::XML::Text` node

```
some_node.replace('this will replace the current node with a text node')
```

closes #115

lib/oga/xml/node.rb

@@ -152,6 +152,27 @@ module Oga
         return node_set.delete(self) if node_set
       end
 
+      ##
+      # Replaces the current node with another.
+      #
+      # @example Replacing with an element
+      #  element = Oga::XML::Element.new(:name => 'div')
+      #  some_node.replace(element)
+      #
+      # @example Replacing with a String
+      #  some_node.replace('this will replace the current node with a text node')
+      #
+      # @param [String|Oga::XML::Node] other
+      #
+      def replace(other)
+        if other.is_a?(String)
+          other = Text.new(:text => other)
+        end
+
+        before(other)
+        remove
+      end
+
       ##
       # Inserts the given node before the current node.
       #

spec/oga/xml/node_spec.rb

@@ -179,6 +179,28 @@ describe Oga::XML::Node do
     end
   end
 
+  describe '#replace' do
+    before do
+      @node      = described_class.new
+      @container = described_class.new(:children => [@node])
+    end
+
+    it 'replaces a node with another node when called with Oga::XML::Node' do
+      other = described_class.new
+
+      @node.replace(other)
+
+      @container.children[0].should == other
+    end
+
+    it 'replaces a node with Oga::XML::Text when called with String' do
+      @node.replace('a string')
+
+      @container.children[0].should be_an_instance_of(Oga::XML::Text)
+      @container.children[0].text.should == 'a string'
+    end
+  end
+
   describe '#before' do
     before do
       @node      = described_class.new