Improve encapsulation on TimedCrowdsale

contracts/crowdsale/price/IncreasingPriceCrowdsale.sol

@@ -49,8 +49,8 @@ contract IncreasingPriceCrowdsale is TimedCrowdsale {
    */
   function getCurrentRate() public view returns (uint256) {
     // solium-disable-next-line security/no-block-members
-    uint256 elapsedTime = block.timestamp.sub(openingTime);
-    uint256 timeRange = closingTime.sub(openingTime);
+    uint256 elapsedTime = block.timestamp.sub(openingTime());
+    uint256 timeRange = closingTime().sub(openingTime());
     uint256 rateRange = initialRate_.sub(finalRate_);
     return initialRate_.sub(elapsedTime.mul(rateRange).div(timeRange));
   }

contracts/crowdsale/validation/TimedCrowdsale.sol

@@ -11,8 +11,8 @@ import "../Crowdsale.sol";
 contract TimedCrowdsale is Crowdsale {
   using SafeMath for uint256;
 
-  uint256 public openingTime;
-  uint256 public closingTime;
+  uint256 private openingTime_;
+  uint256 private closingTime_;
 
   /**
    * @dev Reverts if not in crowdsale time range.
@@ -32,16 +32,30 @@ contract TimedCrowdsale is Crowdsale {
     require(_openingTime >= block.timestamp);
     require(_closingTime >= _openingTime);
 
-    openingTime = _openingTime;
-    closingTime = _closingTime;
+    openingTime_ = _openingTime;
+    closingTime_ = _closingTime;
+  }
+
+  /**
+   * @return the crowdsale opening time.
+   */
+  function openingTime() public view returns(uint256) {
+    return openingTime_;
+  }
+
+  /**
+   * @return the crowdsale closing time.
+   */
+  function closingTime() public view returns(uint256) {
+    return closingTime_;
   }
 
   /**
    * @return true if the crowdsale is open, false otherwise.
    */
   function isOpen() public view returns (bool) {
     // solium-disable-next-line security/no-block-members
-    return block.timestamp >= openingTime && block.timestamp <= closingTime;
+    return block.timestamp >= openingTime_ && block.timestamp <= closingTime_;
   }
 
   /**
@@ -50,7 +64,7 @@ contract TimedCrowdsale is Crowdsale {
    */
   function hasClosed() public view returns (bool) {
     // solium-disable-next-line security/no-block-members
-    return block.timestamp > closingTime;
+    return block.timestamp > closingTime_;
   }
 
   /**