Sign then Encrypt

Ok. Hopefully this addresses all the past feedback. I tried to simplify the PR and still get what we need in. This one is also a prerequisite for #840 as mentioned below. @Renelvon @gubatron take a look.
I 'd like to have the data and the signature built automatically by some function, but this requires refactoring the code. @yagoulas will take care of this. By the way, python will autojoin strings inside parenthesis, so you can use the following style instead of the dreaded backslash:
```python
long_string = (
    "part 1"
    "part 2"
    "part 4"
    ....
    "part 42"
)
```