Changes to be committed:
* src/lparse.ml: "Built-in" is only usable during
	lexp_parse of internal typer files
* .travis.yml: try to make travis compile
* opam: added opam package definition (travis might need it)
* GNUmakefile: add a dummy (make install) for opam

Changes to be committed:
* src/debug_util.ml
* src/lparse.ml:
	- default_lctx is not a function anymore (avoid reading
	builtin files more than once)
	- builtin types are now built from the context
* tests/*
	- adapted to changes
* src/eval.ml
	- eval now uses elexp instead of lexp
* src/util.ml
	- added `debug_msg` which eval whatever it has been
	given as argument if `_typer_verbose` is >= 4