Solidity warnings fixes

[rkolpakov]

All problems with solc warnings were fixed except one:

function now() internal view returns (Timestamp res)
This declaration shadows a builtin symbol.solidity (2319)

In previous versions of Solidity there was block.now() method so Timestamp.now() seems to shadow overrides it. There is no possible solution to drown that warning so possible solutions:

1. leave it as is;
2. change now to something like current.

[Psirex]

I would personally leave it as is. The now() method is deprecated and can't be used, plus the method is declared in the library, not in the global scope.