Refactor clang::tarball recipe and update to clang version 3.3

@michaelklishin @joshk can you please review this?

Note: I give up trying to auto-construct the download URL, because LLVM "packaging conventions" are not stable at all, as illustrated here:

* `clang+llvm-3.3-amd64-Ubuntu-12.04.2.tar.gz`
* `clang+llvm-3.2-x86_64-linux-ubuntu-12.04.tar.gz`
* `clang+llvm-3.1-x86_64-linux-ubuntu-11.10.tar.bz2`
* `clang+llvm-3.1-x86_64-linux-ubuntu_12.04.tar.gz`
* `clang+llvm-3.0-x86_64-linux-Ubuntu-11_04.tar.bz2`

I find therefore quite easier to just update the download link attribute when bumping version... (I added checksum validation, following style of other travis-made cookbooks)

Update to clang 3.3 is related to travis-ci/travis-ci#979