fix: Use double quotes in contract imports (#494)

* fix: Use double quotes in contract imports
* fix: add source location of ENS contracts

Author: sohkai

contracts/lib/ens/AbstractENS.sol

@@ -1,3 +1,5 @@
+// See https://github.com/ensdomains/ens/blob/7e377df83f/contracts/AbstractENS.sol
+
 pragma solidity ^0.4.15;
 
 

contracts/lib/ens/ENS.sol

@@ -1,7 +1,9 @@
+// See https://github.com/ensdomains/ens/blob/7e377df83f/contracts/ENS.sol
+
 pragma solidity ^0.4.0;
 
 
-import './AbstractENS.sol';
+import "./AbstractENS.sol";
 
 /**
  * The ENS registry contract.

contracts/lib/ens/PublicResolver.sol

@@ -1,6 +1,8 @@
+// See https://github.com/ensdomains/ens/blob/7e377df83f/contracts/PublicResolver.sol
+
 pragma solidity ^0.4.0;
 
-import './AbstractENS.sol';
+import "./AbstractENS.sol";
 
 /**
  * A simple resolver anyone can use; only allows the owner of a node to set its