Minor improvements to the setup

I had a look at the current state and noticed a couple of minor things we might want to address:

* Currently there is no test case that covers the multiple files case
* Is there any reason we call the file with the test fixtures `tests` and not `testdata` which is the official name?
* Would be good to have a .gitignore file that contains `vendor` in case someone runs in vendored mode, the `represent` executable that is created when running the command from the readme and any `.exe` files that are created on Windows
* Would be good if the readme also showed an example command that would actually run, I found this very helpful with the testrunner, e.g. `./represent hamming ./representer/tests/hamming/1 .` (not sure how it looks for windows)

cc @tehsphinx