Fix errors in LinkHelper and LinkInlineParser.

[snnz]

Here is what I used to fix #830. LinkHelper.TryParseTitle is replaced with more structured and clearer one, check it out.

[snnz]

I've applied the suggestions.

By the way, since backslashes are already handled by LinkHelper, and there is fortunately that option, removeBackSlash, I didn't touch HtmlHelper.Unscape itself. But it is also used somewhere else, and doesn't look right: 1) it's this function that eats all the backslashes in a sequence except one, and 2) method CharHelper.IsEscapableSymbol that it uses to determine whether to unescape a character or to preserve the backslash, for some reason adds '•' as escapable symbol. Why is that?