Bugfix: Classmap parsing of PHP source with weird escape-like pattern

This PR corrects a bug where the classmap generator gets fouled up on source like this:

```php
$x = "foo\
bar";
```

Specifically, it doesn't like multiline strings where one of the lines ends with a backslash.

It's worth noting that PHP won't actually interpret this as an escaped newline, but instead will just blindly put the backslash *and* the newline into the string. This is probably not what the author expected, but regardless it is valid PHP code.

[Here](https://github.com/openid/php-openid/blob/master/Auth/OpenID/Server.php#L375) is the real-life code that I encountered which exposed this bug to me.