Better handling for svyglm models and fix for #39

[tzoltak]

• [v] if suggesting code changes or improvements, open an issue first
• [v] for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• [v] for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• [v] add code or new test files to /tests for any new functionality or bug fix
• [v] make sure R CMD check runs without error before submitting the PR

This pull requests deals with #39 and #40 .

[leeper]

Thanks or this and sorry for long delay in reviewing. Looks good to me. Can you rebase and resubmit (there are some merge conflicts)? If not, I'll try to get to it.

[tzoltak]

I can do this but within few weeks time.