Makefile: for non-gophers

Merge branch 'rlp-size-validation' of https://github.com/fjl/go-ethereum into fjl-rlp-size-validation

Conflicts:
	eth/protocol.go

Fix p2p node key persistence

crypto.LoadECDSA has been modified to expect hex data.
The key was being saved as raw bytes, causing the file
to be rewritten on every start.

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

If all peers have been tried during the block download process and some
hashes are unfetchable (available peers > 0 and fetching == 0) throw an
error so the process can be aborted.

* Downloader's peers keeps track of peer's previously requested hashes
  so that we don't have to re-request
* Changed `AddBlock` to be fully synchronous