Add possibility to sign and verify JSON strings directly

### Problem

Imagine the following:

* Sign some data on a smartphone, using smartphone libraries
* Trying to verify that in `python-jws`, we first need to parse the JSON and `python-jws` will re-convert the generated object to JSON in its verification process. But if the re-conversion does not format the JSON in exactly the same way as the library on the smartphone did (i.e. the way it was in the payload), verification fails. But the signature was good!

### Solution

Add the possibility to verify a signature on a json *string*, not only python objects.