support sdf3 file endings
fixed ambiguity for empty context-free/lexical syntax sections
SDF-like module system, but without renamings
example w/ module
name binding for modules
do not give errors on missing sort CONTENTCOMPLETE
improved name binding: modules scope sorts, non-transitive imports,
errors and warnings
refactor Stratego generator to create rules with recursive calls in with clauses

Instead of calling prettyprint rules recursively in the build of a Box term, do these
calls in a with clause. This provides more detailed stack traces when attempting to
pretty-print illformed terms.
Merge branch 'master' of https://github.com/metaborg/sdf
added sdf3 to file endings in plugin.xml
Use where clause instead of with because the pp library neds to handle failure in pp rules.