Bouncer contract implementation (#883)

* feat: implement bouncer contracts

* fix: rename Bouncer to SignatureBouncer

contracts/mocks/BouncerMock.sol

@@ -0,0 +1,22 @@
+pragma solidity ^0.4.18;
+
+import "../access/SignatureBouncer.sol";
+
+
+contract SignatureBouncerMock is SignatureBouncer {
+  function checkValidSignature(address _address, bytes _sig)
+    public
+    view
+    returns (bool)
+  {
+    return isValidSignature(_address, _sig);
+  }
+
+  function onlyWithValidSignature(bytes _sig)
+    onlyValidSignature(_sig)
+    public
+    view
+  {
+
+  }
+}