Add test for strings with escaped quotation marks

Signed-off-by: Eric James Michael Ritz <ejmr@plutono.com>
Better configuration of multiline strings

cc-mode was configured to have escaped newlines in string literals and
to not have multiline strings, while it's actually the other way
around. This caused the opening " of a multiline string to get a
warning-face. I changed the test for issue-21 so it will fail in that
case. The test still passes because this commit fixes the issue.

The behaviour of escaped quotes as described in issue-174 is now
different, but not yet fixed.

Signed-off-by: Eric James Michael Ritz <ejmr@plutono.com>
Fix bad highlighting after escaped quotation mark

For keyword fontification to work we need backslashes as part of
words. But when doing syntactic fontification (comments and strings)
backslashes must have escape syntax.

I considered two ways of dealing with this:

1. Make \ word syntax by default and advice
`font-lock-fontify-syntactically-region' to temporarily set it to escape.

2. Make \ escape syntax by default and advice
`font-lock-fontify-keywords-region' to temporarily set it to word.

This commit implemented the second option so we don't change the
behaviour of forward-word compared to current stable php-mode.

Signed-off-by: Eric James Michael Ritz <ejmr@plutono.com>