Extract Converter class

At the moment, it doesn’t do much, but it will perform most of the
conversions in TODO. Note that the quotes are added by the Converter,
not by run, so the Converter can also add other annotations.
Extract convert method
Add tests
Eclipse settings: Explicit UTF-8, Unix line breaks
.gitignore: Ignore .ceylon/config.old
Implement @author → by
Don’t allow description after block tags

Previously, the following

    /**
     * line 1
     * @author me
     * line 2
     */

would be converted into

    "line 1
     line 2"
    by("me")

which isn’t really right. That doesn’t matter right now because
multi-line authors don’t make sense either, but I’m going to add
@throws next, and that can have a multi-line description.