Don't rely on Julia to decompress the artifacts *.bz2

[amontoison]

In #93, I updated the git-tree-sha1 of each artifact stored in Artifacts.toml such we have the correct hash once the archives are decompressed.
The issue is that Julia doesn't handle automatically the decompression of these archives.

We have to do it in this package.
It will simplify the current code and fix #71.