Internal crowdsales (#1439)

* Made some internal crowdsale methods internal.

* Made all Crowdsale constructors internal.

contracts/mocks/CrowdsaleMock.sol

@@ -0,0 +1,9 @@
+pragma solidity ^0.4.24;
+
+import "../crowdsale/Crowdsale.sol";
+
+contract CrowdsaleMock is Crowdsale {
+  constructor(uint256 rate, address wallet, IERC20 token) public
+    Crowdsale(rate, wallet, token) {
+  }
+}