Injector

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- Fixed regression with injecting unbound key resulting in None instead of raising an exception

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- Exception is raised when :class:`~injector.Injector` can't install itself into a class instance due to __slots__ presence
- Some of exception messages are now more detailed to make debugging easier when injection fails
- You can inject functions now - :class:`~injector.Injector` provides a wrapper that takes care of injecting dependencies into the original function

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- Made :class:`~injector.AssistedBuilder` behave more explicitly: it can build either innstance of a concrete class (``AssistedBuilder(cls=Class)``) or it will follow Injector bindings (if exist) and construct instance of a class pointed by an interface (``AssistedBuilder(interface=Interface)``). ``AssistedBuilder(X)`` behaviour remains the same, it's equivalent to ``AssistedBuilder(interface=X)``

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- Auto-convert README.md to RST for PyPi.

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- Added a ChangeLog!
- Added support for using Python3 annotations as binding types.