Add the Fields Command

This patch adds the `fields` command which displays all available
Trackers, Priorities, Statuses and Time Entry Activities for the current
account.

* Add the `fields` command and the printFields function to display
  available tracker, priority, status and activity options.
* Add the Priority data type and the getPriorities and
  getPriorityFromName functions to represent and retrieve Issue
  Priorities.
* Add a Quickstart section and zsh completion information to the
  documentation.

* Modify the `issues` command to allow filtering by Status.

* Remove the getAllIssues API function, as it is a duplicate of the
  getIssues function.
* Remove the getMyIssues API function, you may pass a custom IssueFilter
  to the getIssues function instead.

* Refactor the getIssues function, removing unnecessary get parameters.

* Fix a bug caused blank GET parameters to be sent when filtering the
  `issues` command.
* Fix a bug preventing the usage documentaton from showing if no config
  file was present.