Refactor clang::tarball recipe

 * Renounce to dynamically construct download URL.
   The download link is now defined as a cookbook attribute, since
   llvm.org packages do not follow stable naming conventions.
 * Use 'ark' resource to download and install clang tarball.
[clang 3.1+] Don't symlink to removed 'llvm-ld'

From http://llvm.org/releases/3.1/docs/ReleaseNotes.html:

> The llvm-ld tool has been removed. The clang driver provides a
> more reliable solution for turning a set of bitcode files into a
> binary. To merge bitcode files llvm-link can be used instead.

**KISS Note:**

This cookbook is not (yet) oriented to install "all possible" clang versions.
If wanting to do so, the list of binaries to symlink should be
dynamically constructed based on clang version, or extracted as cookbook
attribute to give more flexibility.
Bump clang version to 3.3
Merge pull request #212 from gildegoma/clang-3.3

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