Feature/fix str repr methods

[ekampf]

Related to issue #5

1. Fix str and repr methods to reflect value being wrapped
2. Add or_none and or_empty_list helper methods
3. Add custom exception when calling get() on a Nothing

[coveralls]

Coverage decreased (-0.7%) to 75.601% when pulling 320756e on feature/fix_str_repr_methods into e055fd9 on master.

[coveralls]

Coverage decreased (-0.9%) to 75.427% when pulling acfb08a on feature/fix_str_repr_methods into e055fd9 on master.

[coveralls]

Coverage decreased (-0.9%) to 75.427% when pulling acfb08a on feature/fix_str_repr_methods into e055fd9 on master.

[coveralls]

Coverage decreased (-0.2%) to 76.109% when pulling b99167d on feature/fix_str_repr_methods into e055fd9 on master.

[coveralls]

Coverage increased (+1.5%) to 77.816% when pulling 444eab0 on feature/fix_str_repr_methods into e055fd9 on master.

[coveralls]

Coverage increased (+1.5%) to 77.816% when pulling 444eab0 on feature/fix_str_repr_methods into e055fd9 on master.