Unify the bootstrap shellscripts into one. Add support to Arch Linux, Ubuntu and OSX. (#221)

* Unify the bootstrap shellscripts into one. Add support to Arch Linux, Ubuntu and OSX.

 o Added a couple of sanity checks to avoid breaking the script
   when something goes wrong (i.e.: when you get a broken llvm
   src tarball).
 o Always attempt to continue if the required packages could not
   be installed, informing the user to make sure that the needed
   libraries are correctly installed.
 o The bootstrap.osx.sh steps have been merged.

* The bootstrap script failed to build when it was not able to find the third-party/llvm/llvm_version file.

This error is triggered when building from a "third-party/llvm"
folder extracted with the previous version of the shellscript,
because the llvm_version file is missing.