Fix warnings

[nventuro]

Part of #1346

With this PR, there is only one warning remaining, which we can't get rid of yet (unless we add a dummy state variable read, but while this may be fine for a mock, I don't think it is for actual production code):

Crowdsale.sol:138:5: Warning: Function state mutability can be restricted to pure
    function _preValidatePurchase(address beneficiary, uint256 weiAmount) internal view {
    ^ (Relevant source part starts here and spans across multiple lines).

It's probably best to review each commit separately.

Regarding the changes:

• Solidity removed warnings on functions with empty body in Supress unused var and state mut warnings for functions with empty body ethereum/solidity#5630, which is part of 0.5.2. This, coupled with the newly added recommendation to use the latest compiler version is IMO enough to switch to 0.5.2, considering it should be a drop-in replacement for 0.5.0.
• There is precedent for an argument to end with an underscore if it matches a public function in PaymentSplitter#_addPayee
• A library that works with data structures being named in plural is consistent with Roles (which works on a Role) and Arrays (which works on an array)

[frangio]

Nice!