Add basic tests.

test/README.markdown

@@ -0,0 +1,21 @@
+## Testing vim-gitgutter
+
+### Run the tests
+
+```sh
+$ cd test
+$ ./test.sh
+```
+
+### Add a new test
+
+1. Add new test function to `test.vim`.
+2. Add a call to the test function in `test.vim`.
+3. Run the tests.
+4. Inspect output from the new test function.  If good, copy it to `<blah>.ok`.
+5. Run the tests to ensure new test's output is verified.
+6. `rm *.out` and commit changes.
+
+### Potential test pitfalls
+
+Currently the tests are order-dependent because the sign id counter isn't reset between tests.