Integrate recent changes from google3.

protoc
- New flags --encode and --decode can be used to convert between protobuf text
  format and binary format from the command-line.
- New flag --descriptor_set_out can be used to write FileDescriptorProtos for
  all parsed files directly into a single output file.  This is particularly
  useful if you wish to parse .proto files from programs written in languages
  other than C++: just run protoc as a background process and have it output
  a FileDescriptorList, then parse that natively.

C++
- Reflection objects are now per-class rather than per-instance.  To make this
  possible, the Reflection interface had to be changed such that all methods
  take the Message instance as a parameter.  This change improves performance
  significantly in memory-bandwidth-limited use cases, since it makes the
  message objects smaller.  Note that source-incompatible interface changes
  like this will not be made again after the library leaves beta.

Python
- MergeFrom(message) and CopyFrom(message) are now implemented.
- SerializeToString() raises an exception if the message is missing required
  fields.
- Code organization improvements.
- Fixed doc comments for RpcController and RpcChannel, which had somehow been
  swapped.