Added support for Binutils trunk.

Removed downloading and expanding actions from build procedure. These
actions now run before build procedure is invoked.
* bootstrap: Changed to invoke downloading/expanding or cloning scripts.
* download.sh: Newly added.
* checkout.sh: Newly added.
* jamroot: Added support for `trunk` argument of `with-binutils` option.
* binutils/jamfile: Removed downloading and expanding actions.
* gcc/jamfile: Added dependency on the timestamp file of Binutils source
  tree to GCC build target.