Add support for pipeline webhooks

[naegelyd]

Webhooks are callbacks over HTTP which enables external services to be notified when certain events happen. When an event occurs, all URLs registered with the event will be POSTed to with the appropriate data. This requires:

• a means to define and store an event and URL (the webhook)
• a means of executing the webhooks when the event occurs

The initial events will include:

• sample loaded

Read more about webhooks on the wiki

Originally reported as chop-dbhi/varify#4