Android: decode images into recycled bitmaps using Ymagine.

This does a good job recycling Android bitmaps, and avoids allocations on
the Java heap.

This will dynamically link to Ymagine, so it requires the Ymagine .so to
be loaded first.
Avoid superfluous changes to the proxy service.
Updated the README.
Android: use a work thread for running response handlers for HURL

We don't want expensive decoders running on the network threads.
Updated README
Leak fix: release thread memory when freeing pool.
Tests: try to trigger memory corruption.
Android: improve init ordering.
Simplified the getter for the URLRequestContext.