Needs

Changes:
- Fixed a bug where combination needs with or as the parent weren't raising the correct exception.

Changes:
- Fixed a bug where errors were not correctly preserved across combinations.

Changes:
- Errors inherit better for combination needs (e.g `with true_need & false_need` will raise `false_need.error`).

Changes:
- A `Need` can be initialized with a boolean (defaults to `True`).
- `Need` casts `is_met()` to a boolean when used as a boolean, so `is_met()` may now return either a boolean or a `Need`.

Changes:
- `needs` no longer wraps the need (for python 2 compatibility).

Changes:
- `Need`s can be used directly as decorators.
- `CombinationNeed` is used directly in place of subclasses and is bound to the `Need` class. This should allow easier subclassing.
- `CombinationNeed.error` and `NegativeNeed.error` are properties rather than being defined at instantiation. This should allow them to be overridden more easily when `Need.combinator` or `Need.negator` are overridden.