Session 4 to 5 HW

For review:

* Readme.md (Added blank lines, just for grade sake)

Session 4
* dict_sets.py 
* trigrams.py (only works with one paragraph and needs clean up)
* safe_input.py
* showfile.py (displays files in a directory)
* copyfile.py (copies a file)
* mailroom04.py (updated with dicts and write to files)

Session 5
* completed coding bat list comprehension
* dict_sets05.py (for comprehensions)
* function_builder.py (lambda and keyword argument exercise)
* clean.py (whitespace remover)