Add support for graceful shutdown of Renderer

[adamjs]

Currently Ultralight can only be instantiated once per process lifetime without full cleanup due to the inherent design of WebKit (thousands of singletons and many long-lived threads in WebCore and JavaScriptCore).

We'd like to support the ability for the library to be created and destroyed multiple times in a single process (with full reclamation of heap memory) without hosting it in a separate child process (not possible on all platforms).

To accomplish this goal we are doing several things:

• Adding a new Allocator API (control and track all heap memory)
• Adding a new Thread Management API (track all threads)
• Modifying WebCore and JavaScriptCore to exit threads and release resources upon library shutdown

(This is already complete, adding issue for 1.4 milestone)

[adamjs]

This feature is complete in 1.4 (with caveat, Allocator API only available in Pro version).