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 <module>
    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.