Add seekable() method to ArchiveFileData class

I've got an `ar` file that contains a `tar.xz` file, and I'm using
Python 2.7.6 with the `backports.lzma` package to try to extract it:

    import arpy
    import backports.lzma as lzma
    ar = arpy.Archive('at_3.1.13-2_amd64.deb')
    ar.read_all_headers()
    fileobj = lzma.open(ar.archived_files['data.tar.xz'])
    tarfile.open(fileobj=fileobj)

This causes the following error:

    AttributeError: 'ArchiveFileData' object has no attribute 'seekable'

I think `backports.lzma` is checking for a `seekable()` method because
in Python 3 streams are `io` objects that have a `seekable()` method.
Really, it shouldn't assume this on Python 2. But since `arpy` also
supports Python 3, I think the best thing to do is to just add a
`seekable()` method to `ArchiveFileData`.