Add shell script to verify output directory structure

[thobson88]

A shell script to compare the directory structure of input & output directories, providing a partial verification that all input data has been processed by alto2txt.

Usage:

./alto2txt-verify.sh /path/to/alto2txt_input/ /path/to/alto2txt_output/

It produces a new directory named alto2txt-verify containing the results of the verification process.

[griff-rees]

Looks ok to me, but was hoping to have a unittest alongside. I've got a partial means of adding that but still getting my head around the existing unittests. I can add some suggestions as another commit so you can see where I've gotten if you like?