Add accounts package and refactor key stores

* Add initial UserAccount and AccountManager structs
* Add NewAccount, Sign and Accounts functions
* Refactor key stores to use key address as main identifier
  while keeping the UUID.
* Use key address as file/dir names instead of UUID

accounts/accounts_test.go

@@ -0,0 +1,19 @@
+package accounts
+
+import (
+	"github.com/ethereum/go-ethereum/crypto"
+	"testing"
+)
+
+func TestAccountManager(t *testing.T) {
+	ks := crypto.NewKeyStorePlain(crypto.DefaultDataDir())
+	am := NewAccountManager(ks)
+	pass := "" // not used but required by API
+	a1, err := am.NewAccount(pass)
+	toSign := make([]byte, 4, 4)
+	toSign = []byte{0, 1, 2, 3}
+	_, err = am.Sign(a1.Addr, pass, toSign)
+	if err != nil {
+		t.Fatal(err)
+	}
+}