Supporting DOS line endings in BATS tests

We've been working on getting Test Kitchen (a testing framework for Chef) working well on Windows. One side effect is we had folks writing BATS integration tests with Windows, and they were getting CRLF line endings. This was causing some havoc with the read delimiter in the preprocessor, and a pretty obscure error.

The fix is to send the raw test source through tr -d '\r' before preprocessing - it's harmless on files without carriage returns, and it doesn't mutate the original test file.

Thanks so much for BATS - I'm stoked to be able to contribute. Pull req includes a test file with CRLF line endings, and a test to ensure we parse it correctly.