Reassign variable of rendered component

The component that gets passed into renderComponent isn't guaranteed to be the
instance that gets mounted. We want to clone the instance.

Unit tests need to reason about the mounted instance. The first code mod changes:

  ReactTestUtils.renderIntoDocument(<identifier>)

into

  <identifier> = ReactTestUtils.renderIntoDocument(<identifier>)

Using this scripts:

  scripts/bin/codemod -m -d ~/www --extensions js \
   '^(\s*)ReactTestUtils\.renderIntoDocument\(\s*([$a-zA-Z0-9_]+)\s*\)' \
   '\1\2 = ReactTestUtils.renderIntoDocument(\2)'

In the second case I do the same for React.renderComponent. However, there are
alot more unnecessary matches so I only codemod if the same identifier occurs
later in the file.

  scripts/bin/codemod -m -d ~/www --extensions js \
   '^(\s*)React.renderComponent\(\s*([$a-zA-Z0-9_]+)\s*?,(.*?\n?.*?\s\2\b)' \
   '\1\2 = React.renderComponent(\2,\3'

And one more for ReactMount.renderComponent used by internals.

  scripts/bin/codemod -m -d ~/www --extensions js \
   '^(\s*)ReactMount.renderComponent\(\s*([$a-zA-Z0-9_]+)\s*?,(.*?\n?.*?\s\2\b)' \
   '\1\2 = ReactMount.renderComponent(\2,\3'

This still matches many unnecessary cases where the second occurance of the
identifier is a redeclaration or comment. But this code mod doesn't hurt in
those cases.

Finally I have to do the same for:

  this.<identifier> = React.renderComponent(this.<identifier>,

This is a common pattern for production code but not tests. Some of these call
sites will likely break when we move to true descriptors.

  scripts/bin/codemod -m -d ~/www --extensions js \
   '^(\s*)React.renderComponent\((\s*)this\.([$a-zA-Z0-9\_\.]+)\s*?,' \
   '\1this.\3 = React.renderComponent(\2this.\3,'
Fix some invalid uses of instances

Breaking this out of the other code mod.
More actionable error message for <tbody> and nested <p>

This bites people all of the time. Until we have a better solution, let's just make the error message more actionable (most people don't know how the DOM
gets unexpectedly mutated).
Fix ref behavior for remounting

I'm thinking that setting up `this.refs` in `mountComponent` is better than `construct`. Followed the same pattern as `ReactComponent.Mixin` and nulling out
the value in `construct` and setting it to its initial value in `mountComponent`.