fix(ServerRendering): execution should be sync

The documentation states that React.renderComponentToString
'uses a callback API to keep the API async', but the
implementation is actually synchronous. In order to maintain
this contract the callback should always be called
asynchronously or be change to a synchronous API.

As per the discussion of pull request 982, the API should
be changed to a synchronous one.
Use lowercase "string" so that the documentation complies with typecheckers
Merge pull request #982 from bripkens/async-tostring

fix(ServerRendering): execution should be async