Big tag set review.

    This contains a large set of changes, mainly a review of the
    tagset. In a nutshell
        - Clearer derivational (IG) boundaries.
        - Distinction between suffixes, and lexical properties.
        - A manual on the way: far from complete, but most (if not
          all) tags are now explained briefly (see doc/manual).
Merge branch 'tagset'
License update: TRmorph is now distributed under LGPL.

    This commit also includes some minor updates to README.