API: Test: Setup basic test infrastructure

- Add a 'expect' utility script that can run simple API tests using clients
  developed for any platform.
- Extend travis build matrix to run API tests using the python client and
  valgrind.

This script can be used to write API tests without having to manage nvim's
lifetime:

- It starts a single nvim instance listening on a known socket
- Invokes the test runner, which should connect to NEOVIM_LISTEN_ADDRESS
- The nvim instance started by the script provides a `BeforeEachTest` function,
  which should be called before each test to reset nvim to a clean state.
- It takes care of shutting down nvim once the tests are finished.

As explained
[here](https://github.com/neovim/neovim/pull/737#issuecomment-43941520), it's
not possible to fully reset nvim to it's initial state, but the `BeforeEachTest`
function should be enough for most test cases. Tests requiring a fully clean
nvim instance should take care of starting/stopping nvim.