support for multiple JIRAs

Fixes #2

_IMPORTANT: This changes the CLI_

From the module it works as described in #4 

From the CLI it works as a repeatable option, e.g.
```
$ tickets --jira user:pass@http://jira.appcelerator.org:ALOY,TIMOB --jira otheruser:otherpassword@http://jira.othercompany.com:KEY
```

That way you can add as many JIRAs as you want.

I've added a `url` option which works like the `jira` option before. This one should be used if you want to use only a single JIRA:
```
$ tickets --url http://jira.othercompany.com --keys TIMOB,ALOY,TC --username user --password pass
```