Modifies 'result' to be 'builder_result', as it is such a common name, t...

Very commonn name for the internal 'result' of a builder.
Changed the property to be named builder_result, so it is really harder that any protobuf field will be named builder_result.

The problem was spotted when compiling Hotpanel protocol:
http://mobiledoc.badoojira.com/_downloads/hotpanel.proto

There are fields in messages called 'result'. These clash with the 'result' in builder, so generated code does not compile.