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.

yamllint/rules/comments.py

@@ -97,7 +97,9 @@ def check(conf, comment):
                     comment.column_no == 1 and
                     re.match(r'^!\S', comment.buffer[text_start:])):
                 return
-            elif comment.buffer[text_start] not in (' ', '\n', '\0'):
+            # We can test for both \r and \r\n just by checking first char
+            # \r itself is a valid newline on some older OS.
+            elif comment.buffer[text_start] not in {' ', '\n', '\r', '\x00'}:
                 column = comment.column_no + text_start - comment.pointer
                 yield LintProblem(comment.line_no,
                                   column,