Add workers_available/0 fun

[varnerac]

Adds a workers_available/0 function to detect if there are bcrypt (nif or port) available. This function is useful for load shedding.

[mworrell]

Hi, nice addition and clean implementation.

One question, could we call it is_worker_available/0 ? Then it is clear that we expect a boolean result.

[varnerac]

I'll make that change.

[varnerac]

Pushed with associated doc changes

[mworrell]

Great work, thank you!