Add getStats()

getStats() allows at any point to retrieve a Dict with the numbers of
successes, failures, errors and failures+errors.

This allows to put a `exit(getStats()["nNonSuccessful"])` as the last
line in `test/runtests.jl` to indicate to the outside world (for
example travis) whether the tests were successful.
Set up travis and add travis build status to Readme

This shows how to use FactCheck.jl with travis