examples: symlink frenetic apps into examples

Use learning and discovery as examples of frenetic applications. Rather
than duplicating code, just symlinking into the examples directory.

Closes #259.
travis: add cmdliner as opam dependency

Committing directly to master to fix build.
(cherry picked from commit 12d3723040571650195d6620c7cb1c699ff6f059)
Merge pull request #282 from frenetic-lang/examples

examples: symlink frenetic apps into examples