add signals for the plugins to be used

mythril/laser/ethereum/plugins/signals.py

@@ -0,0 +1,15 @@
+class PluginSignal(Exception):
+    """Base plugin signal
+
+    These signals are used by the laser plugins to create intent for certain actions in the symbolic virtual machine
+    """
+    pass
+
+
+class PluginSkipWorldState(PluginSignal):
+    """ Plugin to skip world state
+
+    Plugins that raise this signal while the add_world_state hook is being executed
+    will force laser to abandon that world state.
+    """
+    pass