Change function modifiers and condition checking instruction

[chrsow]

There are two changes in the commit

1. Use require() and assert() instead of throw for condition checking

• Since the solidity version from 0.4.10 onwards supports require() and assert().
  Those two function will refunds gas in case of an error occurs.

Ref: http://solidity.readthedocs.io/en/develop/control-structures.html#error-handling-assert-require-revert-and-exceptions

2. Change functions modifiers 'view' and pure instead of constant

• Version 0.4.17 supports view and pure which has more clear leading word than constant.

Ref: http://solidity.readthedocs.io/en/develop/miscellaneous.html#modifiers

[stonecoldpat]

Hey thanks for doing this! I'll test out the changes (and check everything is working with latest solidity) before committing them

[fedorloshakov]

@chrsow, how did you compile smart contract? Because I also tried to compile in Etherium Wallet and received warning message:
Warning: Jump instructions are low-level EVM features that can lead to incorrect stack access. Because of that they are discouraged. Please consider using “switch” or “for” statements instead.
jumpi(end, iszero(bit))
^---^
The same warning for jump assembly command.