User defined JQL queries for Jira services

The user can now change the JQL query which is used for Jira requests.
Add default for jira.query

Default query is the query from the old version and shall prevent
silent fails for bugwarrior users running bugwarrior-pull as cron job
or launchd task.
More elegant setting of jira query variable
Merge pull request #47 from ubuntudroid/develop

User defined JQL queries for Jira services