Added support for larger thrift structs.

We had an issue with the scrooge generated code not compiling for rather large thrift structs with more than 500 fields. It would fail with a method code too large exception.

I have changed the generation so that for structs with more than 500 fields it generates separate decode methods for each field and the case statements in the decode method will call these field specific decode methods. For structs with less than 500 fields the generated code should be unchanged.