Added a first draft for logging work

PR for #10. :)

This PR adds two commands:

`jira worklog <issue>` to see logged work for an issue (without any support for pagination at the moment)
`jira worklogadd <issue> <timeSpent> [comment]` for adding a new worklog record to an issue with an optional comment.

Not sure what about the name of the second command, any suggestions for improvements are welcome!