First commit for unit-testing

- Bundling the 'check' unit test framework. See this as a boot-strap
  step, rather than a library dependency.

This couldn't have been done without first creating 'libettercap' ! 

Right now, I'm only testing out 'get_decoder' in a few cases. I was doing some profiling, and I believe that we can optimize the way that get_decoder actually finds the decoders. With these unit tests, I'll be able to make sure everything works after having made changes. 

See README.TESTS for details