Added a patch for Boost 1.56.0.

* bootstrap: Added commands to apply the patch to Boost 1.56.0 source tree.
* boost/1.56.0.patch: A patch for Boost 1.56.0.
Fixed #88.

* .gitignore: Told to ignore GCC source tree directories with the name like
              `gcc-5-20140921`.
* gcc-release-versions.sh: Fixed extraction and manipulation of version
                           strings. See the description of #88 for more
                           detail.
* compilers.jam: Likewise.
Merge pull request #89 from Cryolite/fix/gcc5

Added a patch for Boost 1.56.0 and fixed #88