Guice Servlet should allow for on-demand instantiation of servlets

_From [mmastrac](https://code.google.com/u/115459243651688775505/) on June 24, 2010 18:10:11_

When using guice-servlet on AppEngine, you can incur a startup penalty on the first request as Guice attempts to instantiate all bound servlets and dependencies, potentially forcing a large number of class loads.

It would be useful if guice-servlet allowed a configuration mode where servlets were loaded on-demand. This would let us amortize the startup costs of the application over the first few requests, rather than paying it all upfront on the first request.

_Original issue: http://code.google.com/p/google-guice/issues/detail?id=505_