Mini-lambda

* the latest `autoclass` has its `__init__.py` fixed, this revealed a `ImportError: cannot import name 'getmembers'` because we were importing `getmembers` from it instead of `inspect`. Fixed

* The init file has been improved so as not to export symbols from other packages. Fixes [6](https://github.com/smarie/python-mini-lambda/issues/6)

* Removed annoying warning message when loading goodies
* Removed useless try/except for goodies that do not need import

* Removed all `None` that were appearing in the goodies_generated.py file
* Now compliant with old versions of `typing` module: `typing.Type` is not imported explicitly anymore.

* Fixed [3](https://github.com/smarie/python-mini-lambda/issues/3) and [4](https://github.com/smarie/python-mini-lambda/issues/4). Generated source code has been removed from version control to avoid this kind of errors in the future.
* Travis script has been equipped with an automatic module import checker to detect such issues earlier next time.

* added alias `make_lambda_friendly` for `Constant`, since it is able to convert anything (constants, functions and classes) to lambda-friendly objects usable in expressions.
* Fixed [2](https://github.com/smarie/python-mini-lambda/issues/2) that was a bug happening when using lambda-friendly methods with non-lambda arguments