v4.0.0

v4.0.0 (#3)

* Incompatible with v2 and v3, but easy to adapt. No explicit inheritance, just decorate profile classes with `@envvar_profile_cls`.
* String values only. No serialisers, no deserialisers. Like environment variables are supposed to be.
* No value or default set means `None` is the value.
* Inherits `collections.abc.Mapping` so you can do `dict(**profile)`
* `list(profile)` returns names of all properties
* ability to create a profile instance without an explicit class declaration with `envvar_profile`.
* `to_dict()` produces a dictionary with strings as keys, as one would normally expect.