Load assignee, reporter, and watchers for issues

I need to know the actors associated with an issue.

`JiraClient.LoadIssue` now loads assignee, reporter, and watchers for an issue. The assignee and reporter are returned as part of the original API call, but in my version of Jira (5.1.8) it only returns metadata about watcher (i.e. '3 people watching'). In the [API docs](https://docs.atlassian.com/jira/REST/latest/#d2e1223) it says full data should be returned, but in order to remain backwards compatible, I make an extra API call in `GetWatchers` (a la `GetComments`) to fetch the watchers and strip the container.

These fields are populated into a new `JiraUser` class with name (username), displayName, and emailAddress. For some reason, emailAddress is not returned for watchers so that remains null.