Check for trailing characters when parsing std::string.

[greggomann]

Thanks for the awesome library, kazuho! This issue came up in my work recently, and I thought I'd offer a patch to address what seems to me to be erroneous behavior:

When parsing a stream, it makes sense to return successfully if a valid JSON value is streamed in, regardless of what comes afterward. When parsing a std::string, however, I think it's appropriate to return an error if any non-whitespace trailing characters are detected. In the case of parse(value, std::string), it only makes sense for the input string to contain a single valid JSON value, and comprehensive error checking should catch if this is not the case. This patch checks for such trailing characters and returns an error if they are present. Thoughts?