Added the support of Cradle.

* .gitignore: Told to ignore `cradle-*` directories.
* bootstrap: Added `--enable-cradle` command-line option.
* jamroot:
  * Added handling of `--enable-cradle` command-line option.
  * Added `<cradle>` and `<cradle-hidden>` features.
* cradle/checkout.sh: New. Added script to checkout Cradle repository.
* cradle/jamfile: New. Added build script for Cradle.