Filter by tags

As we discussed in some email, I'm wanting to allow the ticketing gem to filter what to open tickets for using tags.  The previous tag filters I had added would be an inclusive scoping (site assets + tagged assets).  This is exclusive filtering so it will only open tickets for assets in the configured sites if it has one or more of the tags specified in the config file.

I have not tested this in all cases.  I have tested it using the service desk integration.  With that helper I've tested it
* with sites listed
* without sites listed
* with no tag element in the config 
* with an empty tag element
* with 1 tag
* with 2 tags
* with repeating tags

I ran it through as a first run for the site, and a second run for the site.

Let me know what you think?
I think perhaps we should move the generation of the tag_ids to the ticket_service class - generate it once during initialisation and add it to the options array. That way we don't have to do this for each method call.

What do you think?