Allow previously diffed elements to be updated

[tbranyen]

Removes a mount's transaction state when it is attached to another mount, via Transaction.create. This allows it to be re-used as a mount in future transactions.

FixesGH-314

[tbranyen]

Holding off on merging this since I feel there is probably a better way of solving this. Clearly something is being disconnected when appendChild comes into play. When I do some investigating I find that the previously rendered tree for a DOM node that is appended into a different mount resolves to an isolated DOM Node that isn't attached to anything. This is why the unit test in this PR was failing. When I delete the StateCache in this PR, I'm effectively forcing diffHTML to recalculate and restore the VTree->DOM node relationship to the correct node in the DOM.

[tbranyen]

I noticed the test only fails once the inner span is set to 'Test 2' this means it's not actually a problem that the P tag was previously rendered. It looks to be an issue with rendering the span and it gets detached from the paragraph somehow.

it('will support diffing an element with a StateCache',function(){constp=document.createElement("p");// Create the StateCache on <p/> diff.innerHTML(p,"<span>Test</span>");diff.innerHTML(this.fixture,p);// append p into the fixture div // commenting this out makes the test pass// diff.innerHTML(this.fixture.querySelector("span"), "Test 2"); diff.innerHTML(this.fixture.querySelector("p"),"<span>Test 3</span>");assert.equal(this.fixture.querySelector("span").innerHTML,"Test 3");});});