Change case convention for permissions contract to match solidity style guide, add missing godoc comments, automate signature test

Refactor templating to util, move 'Solidity' types to a more proper abi package, improve documentation of permissions snatives and improve argument and return types

word256: pull tendermint/go-common relating to word256 into eris-db for permissions and eris-mint (TODO)

applied to discuss-eris-gateway branch
https://github.com/eris-ltd/eris-db/commit/4769e410880a6daa7a6d2b5d1d7d3c86137223f2



Signed-off-by: Benjamin Bollen <ben@erisindustries.com>