Fix bug with CRLF in new-lines and require-starting-space

Pound-signs followed by a lone CRLF should not
raise if require-starting-space is specified.

If require-starting-space is true, *and* either:
- new-lines: disbale, or
- newlines: type: dos
is specified, a line with `#\r` or `#\r\n` should
not raise a false positive.

This commit also uses a Set for O(1) membership testing
and uses the correct escape sequence for the nul byte.

If we find a CRLF when looking for Unix newlines, yamllint
should always raise, regardless of logic with
require-starting-space.

Closes: Issue #171.

tests/rules/test_new_lines.py

@@ -40,6 +40,16 @@ class NewLinesTestCase(RuleTestCase):
         self.check('---\ntext\n', conf)
         self.check('---\r\ntext\r\n', conf, problem=(1, 4))
 
+    def test_unix_type_required_st_sp(self):
+        # If we find a CRLF when looking for Unix newlines, yamllint
+        # should always raise, regardless of logic with
+        # require-starting-space.
+        conf = ('new-line-at-end-of-file: disable\n'
+                'new-lines: {type: unix}\n'
+                'comments:\n'
+                '  require-starting-space: true\n')
+        self.check('---\r\n#\r\n', conf, problem=(1, 4))
+
     def test_dos_type(self):
         conf = ('new-line-at-end-of-file: disable\n'
                 'new-lines: {type: dos}\n')