Fix model description crash (issue #12)

Fix issue #12 by adding a custom description method for each model.

Currently the models will print:
* `<OPFAnswer: 0x739c7f0 { id = 8474418; parent = <OPFQuestion: 0x10370d20 { id = 8473613; title = "switch between portrait and landscape"; }>; }>`
* `<OPFQuestion: 0x73f7130 { id = 8414091; title = "boost serialization omit version for a wrapper"; }>`
* `<OPFTag: 0xf69deb0 { id = 3654; name = "fluentvalidation"; }>`
* `<OPFComment: 0xf69f300 { id = 8894930; post = <OPFAnswer: 0x734e9f0 { id = 386364; parent = <OPFQuestion: 0x7344e70 { id = 386341; title = "C# How do I convert ticks to minutes?"; }>; }>; }>`
* `<OPFUser: 0x825b590 { id = 13; display name = "Chris Jester-Young"; }>`

Is there anything else you would want the models to print or shall I go ahead and merge this?

/cc @jesjos @gothma @makxan @tdeekens