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/mocks/MessageHelper.sol

@@ -15,6 +15,7 @@ contract MessageHelper {
   }
 
   function call(address to, bytes data) public returns (bool) {
+    // solium-disable-next-line security/no-low-level-calls
     if (to.call(data))
       return true;
     else