ssl key/certificate update

This pull request changes the notebook code for the RSA key/certificate generation to run the notebook with the secure option. This has been put here as @kcrisman suggested in #238, but I don't actually consider this code to be pulled in the main notebook sources:

1. This code was developped for my particular use in a certain specific context. **It might not have production code quality.**
2. **it is not an essential change an could potentially introduce new bugs in the notebookin a sensible area: SECURITY.**
3. In this version, 2048 bit RSA stuff is generated to prevent future issues like  [The Firefox 33 incident](https://groups.google.com/forum/#!topic/sage-support/zglN_3QLCPo). There are people considering to declare 1024 bit keys insecure. **This 2048 bit keys could be unknown side effects in the current sage notebook implementation.**
4. This pull request **IS NOT necessary to solve the Firefox 33 issue.**