Dependencies

- Allow multiple inheritance for Injector subclasses.
- Evaluate dependencies once.
- Add `use` decorator.
- Allow nested injectors.

- Twelve times speed up.
- Protect from incorrect operations with attribute assignment.
- Deny \*args and \*\*kwargs in the injectable classes.
- Classes can be used as default argument values only if argument name ends with
`_cls`.
- Remove `six` library from install requires.

- Turn into module.

- Dependency assignment and cancellation for `Injector` subclasses.

- Correct syntax error for Python 2.6

- Raise `DependencyError` for mutual recursion in constructor arguments and
specified dependencies.
- Show injected dependencies in the `dir` result.
- Deny to instantiate `Injector` and its subclasses.