Add `nsfw` option to `get_random_subreddit`.

The `over18` cookie is set to '1' when `nsfw` is `True`
Incorporate and clean-up PR#342.
Intentionally use uppercase for random and randnsfw as subreddit names in test.
Good thing I fixed that test in the last commit.
Added valid_redditors helper.
Added a `from_sr` parameter to `send_message`.
Expand on the send_message docstring.
Enable HTTPS everywhere.
Fix test_scope_read_get_submission_by_url test.
Make 3.0a1 version.