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.
Fixed a bug.

* cradle/jamfile: Added dependency of Cradle with `<link>shared` property on
                  Boost with `<link>static` property, because Cradle
                  requires Boost.Exception library that is built only as
                  static libraries.
Added a patch for GCC 4.7.4.

* gcc/4.7.4.patch: New. A patch for GCC 4.7.4 in order to make test cases of
                   Cradle work correctly with GCC 4.7.4.
Merge pull request #106 from Cryolite/feature/cradle

Added the support of Cradle.