useful links
http://solidity.readthedocs.io/en/develop/index.html
http://solidity.readthedocs.io/en/develop/units-and-global-variables.html
https://media.consensys.net/ethereum-gas-fuel-and-fees-3333e17fe1dc
http://www.swende.se/blog/Frontrunning.html
https://karl.tech/learning-solidity-part-2-voting/
https://brilliant.org/wiki/merkle-tree/#
https://www.myetherwallet.com/
https://crypto.stanford.edu/cs251/
https://blockchain.berkeley.edu/workshops.html