Notable changes:

• The serialization logic (for both Marshal and Unmarshal) has been optimized. Testing inside Google demonstrate that the new implementation is about 1.3x to 2.1x faster.
• google/protobuf#272 Unknown fields in proto3 are now preserved.
• #472 JSON marshal and unmarshal now check for proto2 required field.
• #483 GetExtension returns the raw bytes if the ExtensionDesc does not have type information.
• #499 Invalid UTF-8 in string types are rejected during marshal and unmarshal.
• #511 Fields of bad wire types are treated as unknown fields instead of an error.
• #544 The protoc-gen-go generator supports the paths=source_relative option.
• #561 Unknown fields in map entries are now ignored instead of causing an unmarshal error.
• #583 The "import public" feature is now implemented using Go type aliases.