Add a progress bar.

See #5.

Squashed commit of the following:

commit fa7b03867e74d60d58d447e6e71dce02d1bde69b
Author: Spencer Piontkowski <spencer@toasedlime.com>
Date:   Wed Mar 5 15:46:20 2014 -0800

    Style the progress bar.

commit a2aa90eb6306b71a7f78f07670464bc7efc5498b
Author: Spencer Piontkowski <spencer@toasedlime.com>
Date:   Wed Mar 5 10:58:36 2014 -0800

    Working progress bar.

commit ad197a9e2e20bd98b02b34a5d26e3609fcef77c1
Author: Spencer Piontkowski <spencer@toasedlime.com>
Date:   Sun Mar 2 22:28:36 2014 -0800

    Duplicate file is an error.

commit f9c5e759c6678703e6d1e2fd0b4b1438f3dee352
Author: Spencer Piontkowski <spencer@toasedlime.com>
Date:   Sun Mar 2 22:05:35 2014 -0800

    Only append errors to the output.

commit 2964208483a3befc36223bc8703f586eff4db435
Author: Spencer Piontkowski <spencer@toasedlime.com>
Date:   Sun Mar 2 21:51:34 2014 -0800

    Add progress bar and move output text to progress bar.
Improve error handling.

Fixes #8
No attachments in import file

Fix the JS error thrown when WXR with no attachments is loaded.

Fixes #9