Migrate to Psych

Sup works perfectly well for my everyday use. Now I can start on sup2.

This is based on #62. 4 merges in #62 are dropped for a cleaner history. They don't count as contribution on Github and git rank-contributors anyway.

After learning a big dose of Psych, I've made the code goes totally the Psych way:

* implement `init_with` and `encode_with` for classes requiring yamling
* Make use of  Psych.load_tags and Psych.dump_tags in favor of the old YAML.add_domain_type (which is a little more messy IMHO).

And the code supports legacy domain `!masanjin.net` as well. By this way we don't have to load syck to migrate at all. Users can just jump from Ruby 1.8 to 2.0. And the migration process is as easy as load & dump sources.

Besides, I've removed 1.8.7 in .travis.yml. If all goes well this should fix #17 and closes #62.