Can now mirror tarballs in a local directory.

- Can add tarballs to local directory; no longer have to download from source URL.
Fix bug in staging.

- Stage needs to account for symlinks in self.tmp_dir
- Use os.realpath on self.tmp_dir when checking to see if a stage needs to be rebuilt
Fix import bug.
Simplify redundant conditional.
Fix spelling error in tmp dir
Ignore more errors when spidering.
Allow file URLs
Architecture string can contain '.' now.
Fix Mac OS package icon.
Fix bug in make jobs parameter.
Bugfix in clean command.