Add tests and test cases (not all passing yet)

Currently, 12/14 tests pass. But fising them will probably involve changing the plugin code, so I will commit here.
Add another failing test case

I noticed this test case in the wild earlier, and just discovered a note to myself about it.
Inspect shiftwidth() instead of &tabstop in tests
Add script tests/run-fixtures-test

The reason I might want to run only this test is that this is the only test that is easily applicable to older versions of DetectIndent, which have fewer features. I want to use the fixtures test to compare accuracy between the old and new algorithms.

I did some refactoring so that this script did not duplicate much code with run-all-tests.
Use more portable shebang #! line in test scripts

This #! line is preferred: http://stackoverflow.com/a/10383546/578288