Removed extraneous imports
Remove redundant method and simplify mapping rule generation code.
Add additional tests for mapping, covering:
- map_list_field decorator and it's operation
- the situation where a target field that is in the source resource, but a value from a different field is required (see the same_but_different field in test cases for an example of this)
Changed pep8 check to limit line length to 120chars, the default value for PyCharm
Added cached_property decorator for properties that do lazy generation of values that don't change.