Add SafeMath lib for uint8 (#344)

contracts/lib/zeppelin/math/SafeMath8.sol

@@ -0,0 +1,32 @@
+pragma solidity ^0.4.11;
+
+
+/**
+ * @title SafeMath8
+ * @dev Math operations for uint8 with safety checks that throw on error
+ */
+library SafeMath8 {
+  function mul(uint8 a, uint8 b) internal pure returns (uint8) {
+    uint8 c = a * b;
+    require(a == 0 || c / a == b);
+    return c;
+  }
+
+  function div(uint8 a, uint8 b) internal pure returns (uint8) {
+    // assert(b > 0); // Solidity automatically throws when dividing by 0
+    uint8 c = a / b;
+    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
+    return c;
+  }
+
+  function sub(uint8 a, uint8 b) internal pure returns (uint8) {
+    require(b <= a);
+    return a - b;
+  }
+
+  function add(uint8 a, uint8 b) internal pure returns (uint8) {
+    uint8 c = a + b;
+    require(c >= a);
+    return c;
+  }
+}