Run dialyzer by default when compiling. Update travis builds.

There are a few changes in this pull request.

 - rebar compile treats warnings as errors. There are no warnings in the code so this is just a future precaution.
 - The dialyzer target in the Makefile included a -Wunmatched_returns option which generated a few extra warnings. These were fixed, mostly by adding "ok =" to a few lines.
 - The "all" target in make has been changed to run dialyzer after a compile. There is a new "compile" target that just does a compile, similar to what the "all" target did up to now.
 - I told travis to run on this branch, and based on this made a few travis changes.
   See https://travis-ci.org/apauley/ibrowse