conda/conda-build — pull request #122 by mutirri (2014-06-12 17:35:21+00:00)
Title: Added external support for tar if std tarfile module can't handle the archives

The standard python module tarfile is unable to always handle archives.
During creation of conda recipe for apache-maven (required by newest spark package) I wasn't able to unpack the archive:
http://ftp.ps.pl/pub/apache/maven/maven-3/3.2.1/binaries/apache-maven-3.2.1-bin.tar.gz

The error:
```
Traceback (most recent call last):
File "/home/irritum/anaconda/bin/conda-build", line 5, in
sys.exit(main())
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/main_build.py", line 71, in main
args.func(args, p)
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/main_build.py", line 177, in execute
build.build(m)
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/build.py", line 181, in build
source.provide(m.path, m.get_section('source'))
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/source.py", line 194, in provide
unpack(meta)
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/source.py", line 60, in unpack
tar_xf(src_path, WORK_DIR)
File "/home/irritum/anaconda/lib/python2.7/site-packages/conda_build/utils.py", line 41, in tar_xf
t = tarfile.open(tarball, mode)
File "/home/irritum/anaconda/lib/python2.7/tarfile.py", line 1665, in open
raise ReadError("file could not be opened successfully")
tarfile.ReadError: file could not be opened successfully
```
With this patch everything works great.