Fixed width format

This is a first try. It's only an idea I'm working and I'm wondering if it would be useful. It's related to parse fixed width data files, and then export to other formats.

For now I'm using [struct](http://docs.python.org/2/library/struct.html) and only as a string. If you want to get a float or integer you can use [`add_formater`](http://docs.python-tablib.org/en/latest/api/)

Any suggestions?

I've been added the tests.