Update docs and add option to silence url output

This pull request includes 3 small changes:

1. Add argument `silent` to `yhat.predict()` and `yhat.predict_raw()`, which gets passed along to `yhat.post()`. The default is `silent = FALSE`, in which case everything stays the same as it is now. Setting `silent = TRUE` suppresses printing of the url to the console in `yhat.post()`.
2. Use `message()` instead of `print()` everywhere so that the user can silence ALL output by, for example, wrapping a call to `yhat.predict()` with `suppressMessages()`.
3. The diffs on all of the .Rd files are due to a newer version of roxygen2.