Relax constraint on requiring API key. Fix panic.

This release relaxes the constraints on when you are required to include an API key before an event will be sent. Previously, all events required an API key, even if they were not going to use it because the configured Sender did not need it. (For example, if you use the DiscardWriter Sender, all events will be dropped. Clearly an API key should not be required. Similarly a sender that sends things to STDOUT or to a non-Honeycomb service may not need an authentication token.) API key is now only required if you are using the default Honeycomb sender or the Mock sender for tests.

This change also fixes a minor bug that could cause a panic if libhoney is not configured before use.