Add 'Appendable' as a destination for Gson

[GoogleCodeExporter]

Currently, Gson can generate a String or write to a Writer.  This can result in 
unnecessary temporary 
String objects when the user wants to embed Gson in a larger response.  Both 
StringBuilder and 
Writer share a common interface however: Appendable.  Since it is unlikely that 
the flush and close 
methods are called on Writer while serializing, consider either adding 
Appendable as a destination 
or replacing Writable with Appendable as the stream output type.

Original issue reported on code.google.com by complexm...@gmail.com on 1 Oct 2008 at 9:32