Chars

I added some more convenience chars for parsing a character from a set of characters.

So for instance, instead of having to use:

``` c#
Parse.Char('a').Or(Parse.Char('b')).Or(Parse.Char('c'))
```

You can use:

``` c#
Parse.Chars('a', 'b', 'c');
// Or
Parse.Chars("abc"); // Using strings
```