Support call functions at block number

[vinhnd2010]

Pass BlockNumber value at the last of params list. Ex:

# creation of contract object
myContract = web3.eth.contract(abi);

# initiate contract for an address
myContractInstance = myContract.at('0x2ad180cbaffbc97237f572148fc1b283b68d8861');

# call constant function at block_number
result = myContractInstance.balanceOf([address], [block_number]); 
puts result 

[vinhnd2010]

@astudnev Please have a look. This PR solves the issue #16. Thanks!