Feature #38 add config file

[ojob]

This is the first commit to implement #38: addition of an options.yml file, that makes parameters persistent on user filesystem.

User can modify the file then restart PhotoCollage; or user can modify the parameters through the settings dialog, then at exit, PhotoCollage will update the settings file.

This commit has been tested with python 2.7 and 3.5, and also with flake8 . and unittest, with no warning/error reported.

[ojob]

Seems that I can't get the newlines right, according to flake8 rules I201 and I202, whatever I try to do following my reading of the documentation.

If anyone gets how I can make this pass the checks, I'm all ears!