Update condition checking and functions modifiers.

[chrsow]

There are two changes in the commit

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

• use require() and assert() instead of throw. These two function will refunds gas in case of an error occurs while throw don't refunds. 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

• use view and pure modifier which has more clear leading word than constant. Ref: http://solidity.readthedocs.io/en/develop/miscellaneous.html#modifiers

[descampsk]

Thanks for this commit.

I won't merge atm, because I didn't manage to compile with Ethereum Wallet with a compiler higher than 0.8.10. Indeed, with all compiler higher than 0.8.10, there is a warning due to the jump instructions which denies me to compile and push the contracts.

And in 0.8.10 version, the key word "pure" doesn't exist. I keep this pull request to merge, when this issue will be resolved.