Various fixes and formatting chores (#885)

* fix: clean up solium linting errors

* fix: make various contracts natspec compliant

* fix: this.balance deprecated; convert to address(this).balance

* fix: contract.call deprecated and switch to gasleft()

* fix: ignore empty block rule project-wide

* fix: add ignore cases for the rest of the linting warnings

contracts/DayLimit.sol

@@ -61,6 +61,7 @@ contract DayLimit {
    * @return uint256 of today's index.
    */
   function today() private view returns (uint256) {
+    // solium-disable-next-line security/no-block-members
     return block.timestamp / 1 days;
   }