store methods, optional serializer, PEP8 file_obj

Created by: fractaledmind

I have made three alterations to version 1.7:

1. I changed all open ... as file: to open ... as file_obj to avoid pylint error when redefining built-in
2. Added a new optional parameter to Workflow() intialilzation:
   • serializer defaults to pickle but can be set to anything that supports load and dump methods
   • self._serializer loads or dumps store and cache data in respective methods
   • stored/cached data is saved in file with extension matching serializer's name (__name__ property)
3. Added stored_data and store_data methods, which mirror cached_data and cache_data in functionality.

Thoughts?