Add RS* signing support

[letmaik]

Implements #21.

Some notes:

• The signing key is passed as PEM string. This could be extended to support DER if a buffer is given. Is this reasonable?
• I also validated signed messages manually using pycose.

TODO:

• README?

[coveralls]

Coverage increased (+0.09%) to 89.912% when pulling 6f8ca48 on letmaik:letmaik/rsa-sign into 3154ca4 on erdtman:master.

[letmaik]

@erdtman The examples submodule doesn't have RS* test cases so I added one in here for now. This can be removed once cose-wg/Examples#105 is done. Let me know if the rest of the PR is fine, happy to make further changes.

[erdtman]

Looks good to me.
+1 yes it would be nice to put an RSA example in the readme (maybe it is time to create a matrix with what is supported also in the readme file)
love that you have submitted a PR to the cose-wg examples too.