Speaker Availability Should be Available via API

[oxinabox]

Problem you are facing

I cannot find out what days speakers are available via the API.

Possible Solution

Add availability field to /speakers endpoint results.

Context

Two reasons:

1. I need to know which speakers are available on which days, so I can work out where to put their talks in the program. Because I am doing my first pass on exported data, I want to do that via the API. Then I can both be informed, and programatically check my early program schedual.
2. For statistical purposes we wish to know how many speakers are staying for the full conference