I forgot to update one instance of "go-ethereum" in commit 3f047be5.

All code outside of cmd/ is licensed as LGPL. The headers
now reflect this by calling the whole work "the go-ethereum library".

crypto/curve.go is not our code and has its own license. This commit
excludes it in update-license.go and removes our GPL header.

This version is less clever. All names are listed in a single file,
AUTHORS. All source files have the same header. This is an improvement
over the previous version, which attempted to list copyright holders in
each source file.

This ensures that compiler error messages contain the correct path.

Many people need or want to build go-ethereum from the git repository,
mostly to stay up to date with recent changes. We cannot expect that
people without Go experience grok the Go workspace concept.

With the Makefile, building from github requires only
three steps (provided that a Go toolchain is installed):

    - git clone https://github.com/ethereum/go-ethereum
    - ... install C libraries (libgmp, etc.) ...
    - make