Fixed - added new '/schedule/availability' endpoint to the API

[valtersg]

Implementation for the feature request of #305

Takes in either:

• start and end (ISO 8601 timestamps)
  or
• next (ISO 8601 duration)

returns:

{
  'availability': <float>,  # fraction of the time range when the server is free from exclusive tasks
  'start': <ISO 8601 timestamp>,
  'end': <ISO 8601 timestamp>
}

[mfeit-internet2]

Looks good. I like what you did with the query.