Add Filter and Sort Options to the Issues Command

This patch removes the project argument from the `issues` command,
replacing it with flag-based filtering and sorting.

* Add detailed documentation to the `stopwork --help` command.
* Add the getIssues API function to retrieve an optionally filtered list
  of Issues.
* Add the Tracker data type and the getTrackers and getTrackerByName
  functions to represent, retrieve and find Redmine Issue Trackers.

* Modify the `issues` command to display all issues with optional
  filtering, instead of issues for a specific project. Filtering is
  available for projects, statuses, trackers and assignees.

* Remove the getProjectsIssues function from the Redmine module's public
  API and the printProjectsIssues action from hkredmine. These can be
  replicated by modifying an IssueFilter before passing it to the
  getIssues API function.