Enable -Wunderspecs in Makefile.

Underspecs ensure that dialyzer will warn about types that are underspecified:
the specification allows a wider set of types than dialyzer can infer. Enabling
this _also_ seems to catch errors that are not set width so much as just not
applicable, [[spec_fun()]] for the desired [spec_fun()] in the last commit.

Had -Wunderspecs been enabled the last bug would not have gone through.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Create an example and update the README.

The README bothered me for being damn near unreadable. The REPL bit is still
hard, but now there's some example code and _that_ at least is more simple to
read.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>
Merge pull request #4 from blt/enable_underspecs

Enable -Wunderspecs in Makefile.
Merge pull request #5 from blt/better_examples

Create an example and update the README.
Update README.md