Base wallet implementation (#35)

* Initial draft of the neo-go wallet

* Cleanup + more test for util package

* integrated wallet into neo-cli partially

* base wallet implementation + smartcontract code.

pkg/util/array.go

@@ -0,0 +1,14 @@
+package util
+
+// ArrayReverse return a reversed version of the given byte slice.
+func ArrayReverse(b []byte) []byte {
+	// Protect from big.Ints that have 1 len bytes.
+	if len(b) < 2 {
+		return b
+	}
+	dest := make([]byte, len(b))
+	for i, j := 0, len(b)-1; i < j; i, j = i+1, j-1 {
+		dest[i], dest[j] = b[j], b[i]
+	}
+	return dest
+}