[PULL] Optimize header dependencies; improve Makefile dependency graph

[jordanlewis]

https://github.com/bitcoin/bitcoin/pull/226

This patch set includes the following changes:

    Remove unused #includes

    Factor out #includes that are only needed in some source files from headers.h

    Add dependency tracking to unix, mac, and mingw makefiles via gcc's -MMD flag

These changes improve bitcoin's development environment. Previously changing any header file caused a global recompile. With these changes, only those files that directly depend on a changed header file will require recompilation.