Makefun

`compile_fun`: added capability to disable recursive compilation (`recurse` arg) , and to exclude some names from compilation (`except_names` arg). Fixed [49](https://github.com/smarie/python-makefun/issues/49) and [#50](https://github.com/smarie/python-makefun/issues/50)

Fixed issue `ValueError: Cell is empty` with `compile_fun`. Fixed [48](https://github.com/smarie/python-makefun/issues/48)

Now raising an `UndefinedSymbolError` when a symbol is not known at compilation time. One step towards [47](https://github.com/smarie/python-makefun/issues/47)

New goodie `compile_fun` decorator to `compile` a function so that it can not be navigated to using the debugger. Fixes [46](https://github.com/smarie/python-makefun/issues/46)

`add_signature_parameters` now accepts that one specifies a custom index where to insert the new parameters.

Added `__version__` attribute to comply with PEP396, following [this guide](https://smarie.github.io/python-getversion/#package-versioning-best-practices). Fixes [45](https://github.com/smarie/python-makefun/issues/45).

Fixed `six` dependency: also declared as a setup dependency.

Added missing `six` dependency explicitly.