contract: externalize build using makefile

New versions of web3 don't include compilation of contract,
that we also might want to compile separately.

Adapt the node to the new API by calling the solidity compiler
directly and move the contract code and new build system
to the contract directory.

Change-Id: I341034c0e7227d78881e6fb8885eb2de10ee03ea
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>