Added a new ResettableInterface and implemented it where possible.

When one use Monolog in a long process like an AMQP worker with a
`FingersCrossedHandler` or `BufferHandler` there is a drawback: as soon as there
is an AMQP message that generate a log >= error (for example), all next AMQP
messages will output logs, even if theses messages don't generate log where
level >= error.

In the same context there is a drawback for processor that add an UUID to the
logs. The UUID should change for each AMQP messages.

---

This patch address this issue with a new interface: `ResettableInterface` interface.
Side note: `reset()`, `flush()`, `clear()`,  are already used in Monolog. So
basically, one can use the `reset()` on the `Logger` and on some
`Handler`s / `Processor`s.

It's especially useful for

* the `FingersCrossedHandler`: it `close()` the buffer, then it `clear()` the buffer.
* the `BufferHandler`: it `flush()` the buffer, then it `clear()` the buffer.
* the `UidProcessor`: it renew the `uid`.

tests/Monolog/Handler/FirePHPHandlerTest.php

@@ -21,7 +21,7 @@ class FirePHPHandlerTest extends TestCase
 {
     public function setUp()
     {
-        TestFirePHPHandler::reset();
+        TestFirePHPHandler::resetStatic();
         $_SERVER['HTTP_USER_AGENT'] = 'Monolog Test; FirePHP/1.0';
     }
 
@@ -77,7 +77,7 @@ class TestFirePHPHandler extends FirePHPHandler
 {
     protected $headers = array();
 
-    public static function reset()
+    public static function resetStatic()
     {
         self::$initialized = false;
         self::$sendHeaders = true;