Rename module to 'SwiftProtobuf'

It turns out there are other 'Protobuf' modules in
the world.  Since we're already calling this the
"Swift Protobuf" project, we may as well use that
name throughout.

Apologies for the disruption.

Test "option (apple_swift_prefix)", fix it.

There's a bug that extension fields were not correctly handled
in nested submessages (the extension set data was not always
passed down to generated subdecoders).

API Break: Use `Data` type from Foundation for binary data

This changes the current API and will break existing clients.
The protobuf binary serialization methods have changed to use Data:
    ProtobufMessage.init(protobuf:) now takes Data instead of [UInt8]
    ProtobufMessage.serializeProtobuf() now returns Data instead of [UInt8]

The [UInt8] support is still available under a different name:
    ProtobufMessage.init(protobufBytes: [UInt8])
    ProtobufMessage.serializeProtobufBytes() throws -> [UInt8]

Similarly, proto `bytes` fields that were previously rendered
as [UInt8] properties in Swift are now rendered to Data properties.

This change was made to better align Swift Protobuf conventions
with Swift 3 Foundation.

Initial commit of version 0.9.13