Create slycat.web.server.plugin.Manager.
Begin loading plugins at server startup.
Begin stubbing-out an API to register model plugins.
Implement the generic model plugin.
First pass at returning HTML from a plugin model.
Embed plugin-model HTML into a standard model template.
Decouple HTML generated by a model plugin from the container HTML template.
Correct two errors in the regression test suite.
Merge branch 'plugin-models'.  Closes #126.
Merge branch 'master' of https://github.com/sandialabs/slycat