add a jql property to Issue that provides an enumerator of the JQL results

Adds a jql property to Issue that returns an Enumerator instance. EG:

~~~
client.Issue.jql("type = Epic").each do |src_issue|
  puts "issue key is: #{src_issue.key}"
end
~~~

will iterate through all the issues matching "type = Epic" and print out the key of each.