Attempt to avoid problems on CRAN if no arXiv connection

If the arXiv API is down, tests at CRAN give errors. Attempting to avoid this.

- Add a function to test for connection to arXiv
- If no connection, skip the rest of the tests
- Add `\donttest{}` to all examples
- If no connection in building vignette, abort early
- Include pre-built vignette and add `BuildVignettes: no` to `DESCRIPTION` file.
- `R CMD check` will still give an error when running vignette code, but it should be more clear.