Rename proposals directory to drafts and move some contracts there (#1271)

* rename proposals directory to drafts directory

* move TokenVesting and SignatureBouncer to drafts/

* rename BouncerMock.sol to SignatureBouncerMock.sol

contracts/drafts/TokenVesting.sol

@@ -2,9 +2,9 @@
 
 pragma solidity ^0.4.24;
 
-import "./SafeERC20.sol";
-import "../../ownership/Ownable.sol";
-import "../../math/SafeMath.sol";
+import "../token/ERC20/SafeERC20.sol";
+import "../ownership/Ownable.sol";
+import "../math/SafeMath.sol";
 
 
 /**

contracts/mocks/ERC20WithMetadataMock.sol

@@ -1,7 +1,7 @@
 pragma solidity ^0.4.21;
 
 import "../token/ERC20/ERC20.sol";
-import "../proposals/ERC1046/TokenMetadata.sol";
+import "../drafts/ERC1046/TokenMetadata.sol";
 
 
 contract ERC20WithMetadataMock is ERC20, ERC20WithMetadata {

contracts/mocks/SignatureBouncerMock.sol

@@ -1,6 +1,6 @@
 pragma solidity ^0.4.24;
 
-import "../access/SignatureBouncer.sol";
+import "../drafts/SignatureBouncer.sol";
 
 
 contract SignatureBouncerMock is SignatureBouncer {