Missing ability to create RsaParameters, which prevents using modulus size 4096

[wellcaffeinated]

Hi there
Forgive me if i've missed something but I'm currently attempting to write a library using ring, and I want to support RSA 4096. However there is no constant defined to allow this. I thought I'd be able to just create the necessary RsaParameters directly, but there is no ability to do this because the properties are private.

If there is no workaround, I'd suggest one of the following:

1. Make the properties pub
2. implement a new() or try_new() method.

Thanks!