Adds structured docs for variable declarations.

- adds natspec generation for state variables.
- exports structured docs for state variables to JSON.

test/libsolidity/syntaxTests/natspec/docstring_variable.sol

@@ -0,0 +1,13 @@
+contract C {
+    function f() public pure returns (uint) {
+        /// @title example of title
+        /// @author example of author
+        /// @notice example of notice
+        /// @dev example of dev
+        /// @param example of param
+        /// @return example of return
+        uint state = 42;
+        return state;
+    }
+}
+// ----