Proposal: Re-add travis configuration and badge

CI configuration was removed in @cf1dc613177 due to nondeterminism caused by travis. This PR introduces the `DISABLE_NONDETERMINISTIC_TESTS` ifndef guard to disable tests that may be affected by nondeterminism in the test environment.

Running the tests in a local ubuntu 12.04 virtualbox, I managed to get some random tests failures in the `tcp_writealot` test. The failures became frequent after a while and persisted even across reboots! It seems the operating system is saving some kind of state related to the network stack that is causing this problem.

I'm not sure what tests were randomly failing before, but for now I only wrapped `tcp_writealot` in the ifndef guard.

If this PR is accepted, I will send another that uses mingw/wine to cross-compile/test for windows. This pattern can be extended to other supported platforms through the use of prebuilt virtual machines and qemu