Fix Issues #24 and #25

[pebreo]

This is my pull request to fix issue #24 and #25.

Issue #24 was about changing the syntax f = open(..) to with open(..) as f

Issue #25 was about generating the *.xls files into their own subdirectory tests/generated_files

A bunch of *_tests.py have been affected but the non-test files that have been affected are:

document_writers.py
textwidths.py

All tests have passed as a result of these changes.