BUG/ENH: Warn when falling back to python parser

- Warn when falling back to python parser due to C-unsupported options
  (via new class ParserWarning).
- Copy tests that previously fell back to python from ParserTests to
  TestPythonParser and assert that they produce ParserWarnings when run
  under other engines.
- Fix C parser handling of leading whitespace and \r-delimited files
  (add test)
- Translate sep='\s+' to delim_whitespace=True (add test)
- Specify engine='python' in tests with C-unsupported options to avoid
  warnings
- For now specify engine='python' in tests with sep='\s+' and
  multi-index columns in input (see GH 6893)