Added new string converters and deprecated old ones.

This addresses #150.
Left out rare conversions for now, as some of them may be ambiguous regarding null termination and I didn't feel like typing more.
Yes, that I probably do. Don't know where the extra plus sign came from.

On Thursday 19 June 2014 00:29:12 Mathias wrote:
> > @@ -673,6 +680,42 @@ char* name = NanCString(args[0], &count);
> > 
> >  delete[] name;
> >  ```
> > 
> > +<a name="api_nan_ascii_string"></a>
> > +### NanAsciiString
> > +
> > +Convert a `String` to zero-terminated, Ascii-encoded `char *`.
> > +
> > +```c+++
> 
> You mean "c++", right? Same some lines below.
> 
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