Add solhint for linting

Author: justinkchen

.solhint.json

@@ -0,0 +1,8 @@
+{
+    "extends": "default",
+    "rules": {
+        "indent": ["error", 2],
+        "quotes": ["error", "double"],
+        "max-line-length": ["error", 120]
+    }
+}

contracts/Migrations.sol

@@ -1,4 +1,4 @@
-pragma solidity 0.4.18;
+pragma solidity ^0.4.19;
 
 
 contract Migrations {

contracts/SetToken.sol

@@ -1,4 +1,4 @@
-pragma solidity 0.4.18;
+pragma solidity ^0.4.19;
 
 
 import "zeppelin-solidity/contracts/token/ERC20/StandardToken.sol";

contracts/lib/Set.sol

@@ -1,4 +1,4 @@
-pragma solidity 0.4.18;
+pragma solidity ^0.4.19;
 
 
 /**