BUG: Warn when falling back to python parser

Warn when falling back to the python parser due to unsupported options.
Copy many tests that previously fell back to python from ParserTests to
TestPythonParser and assert that they produce ParserWarnings when run
under the C engine.
BUG: Fixes to C parser handling of whitespace

BUG: Fix C parser handling of leading whitespace
ENH: Translate sep='\s+' to delim_whitespace=True
TST: Test for correct behavior with leading whitespace
TST: Test for bug with \r-delimited files
TST: Merge tests with sep='\s+' back into ParserTests